The way I do the individual desserts (cheesecakes, key lime pies, tiramisu, choc mousse, etc) is to make the biscuit in full sheet pans. I can get 40 3” circles out of a sheet. I spray the rings (the rings I have are 3” round and 1.5” high), cut the biscuit circles and the put the rings with the biscuit in on a pan lined with parchment (helps with cleanup).
Then I put the cheesecake batter or key lime filling, or mousse, or whatever in. With the cheesecakes, I use an ice cream scooper (aka a disher) to fill, with the mousse sometimes I use a bag and sometimes I just pour it in; with the key lime filling, I usually pour because it’s viscous enough.
So I’m not really wasting anything except the excess of the biscuit, and sometimes I can fit the scraps together but I have to be careful because those are the ones that will leak.
If I am making a layered dessert, I cut the other rings of biscuit and fit them in as I am layering the filling. You put the filling in, then a cut circle of biscuit, then tamp it down (this is important, otherwise you will have bubbles and gaps that look unsightly in the finished dessert), put another layer of filling, then another circle of biscuit, tamp again, and then when it is chilled, add the topping. To unmold, I use a torch to release the sides. You can also use a hair dryer (do not ask me how I know this works - put the chilled mold on something just narrower than the dessert (like a can, or the plastic top of a spray can - be careful if you are using the torch with this, it can melt or catch fire if you are not very careful. The biggest mistake I see people making with torches is they are not aware of the flame and what it is pointing at.)
If for the Opera, you are doing it for just a few people, you might want to do them as AnnieMacD suggests - make a square cake, torte, fill and then cut individual squares instead of making rounds. This way there’s no waste at all, and a lot of the classic French desserts were originally made in squares rather than rounds (aka entremets).