How to proceed when you have a marinade of beef in spices, tomato, vinegar and onion? I followed this recipe of beef vindaloo, last night, triple checking everything written there. Since I don't have a spice grinder, I put the chopped onions, chopped tomatoes, salt and spices in a mixie and converted it into a fine paste. Then poured it into a container which had 1kg cleaned beef pieces, poured 250ml of vinegar (the recipe says 250ml for 0.5kg beef, but I felt pouring 500ml for 1kg of beef is too much).  

It's been marinated in a fridge overnight. Today morning the beef seems a bit tough when I squeeze it. I thought it's supposed to turn tender.  

Anyway, now the recipe asks me to use the marinade for frying the beef a bit. This confuses me.  


How is beef going to get fried (even though there will be 8tbsp of oil), when the marinade contains a good amount of water from the tomatoes and onions?  
Every other recipe I have known, the tomatoes are only added after the frying takes place. I've never known tomatoes being used in marination. Is this really something that works?  
Every other recipe calls for the onions to be fried along with the spices. Is onion really blended into a paste and used for marination?  
Now that I have a rather large quantity of marinade full of onion and tomato, will that ruin the taste of the curry because the onion paste might not actually get fried?  
Should I (instead of trying to fry the marinade), take some fresh onions, chop them up with some ginger and garlic and fry them in oil until golden brown, and take the beef pieces separately and fry it for a while (until the outsides are brown) with the onions, ginger and garlic and later add the entire marinade to make a curry?  


Somehow I feel that if I just follow the rest of the recipe on that page, the curry will get messed up.
tl;dr there is more than one way to cook food, it sounds like you're used to a specifc pattern, but it's not the only one. I'd personally encourage you to try the recipe as-is before you leap to judgment.

(1) It says sauté, not fry, and that will work fine. You should probably shake off excess marinade, and avoid crowding the pan too much. The water will boil and spatter, but the meat will cook and brown. This will be easier if you use the right amount of liquid, though. I see that you reduced the amount of vinegar. You should probably have stuck to the recipe, or replaced the vinegar with water if you wanted to reduce the vinegar flavor.

(2) Sure, you can use tomato sauce/paste in a marinade if you want. It'll impart a bit of tomato flavor to the meat, maybe. The recipe also isn't 100% clear, but does later say "add the reserved masala water", so I think it's also having you use the marinade as the sauce, and the tomato will certainly be obvious enough there.

(3) Yes, you can also use raw onions in marinades. Same note here about how it appears it also ultimately gets cooked into the dish.

(4) It's hard to answer, because I don't really understand why you think this would ruin the dish. The meat has some raw tomato and onion in a marinade, then it gets cooked. You also put raw tomato and onion into the dish, and then cook them. Everything ends up cooked. Not fried, but frying is not the only way to cook food.

(5) I mean, you could do that, and it'd probably be good too, but that's not what this recipe says to do, so it'd be a somewhat different dish. One of the main characteristics of this recipe is that it produces a smooth sauce, with all the flavors blended together: all the spices, the tomato, the onion, everything. There are no noticeable pieces of onion (or garlic or ginger), but the flavor's in it.

(and as for marinades tenderizing things - that's not always the point, not all meat really needs it, but tenderizing also doesn't really mean the meat will be incredibly soft while still raw, just that it'll be more tender and easy to bite/chew)