How to make a custard (royale) that sets up firm on the stovetop? I was trying to make royales for consomme for the first time, since I figured I already have spare yolks already. Following a recipe for custard in general that said one can use as much as 4 yolks per cup of custard to make it "firmer", I used the 7 yolks I had left with 1.5 cups of milk - more precisely, 125 g of yolk to 375 g of milk, and some salt, pepper, and chives.

I put this in a bowl over a pan of boiling water and whisked slowly, but the mixture never seemed to thicken significantly until it began curdling. (I tried to save this by immediately dumping the mixture into a pan and whisking out the curds as much as possible, which might be fine enough for slicing into soup, but I can't tell until this chills.)

Is there a method to follow to end up with firm, sliceable custard by double-boiling on the stove? I'd rather not have to fire up the oven for this. I also do have access to an instant-read thermometer, if stopping at a given temperature range will get me the proper result.
A custard royale is not stirred during cooking, this leads exactly to the scrambeled eggg effect you had. This is fundamentally different from the standard custard process you are probably familiar with.

The (preheated) cream is mixed with the egg and/or yolks, then put in the vessel it's supposed to be cooked in.

It can be cooked over barely simmering water or in the oven in a bain marie at 90C. You simply give it enough time to solidify, which may be somewhere between half an hour to almost an hour, depending on the thickness. Cooking in an oven tends to take a bit longer than on stovetop.

Let cool completely before unmolding and cutting.