The theory all seemed sound, and I was pretty confident of my flavor profile. I cut the leftover lamb into bite-sized chunks, and browned it in butter with pepper, coriander and a bit of salt. (The combination from Fidaush, one of my favorite period lamb dishes.) Then I made a roux out of boiled-down lamb jello and milk, thickened with flour over low heat until it was a reasonably good-looking white gravy. Put the meat and some frozen veggies into the pot, pour the gravy over, cover with a (commercial) crust, and bake until the crust starts to brown.
The flavors were good, and worked as planned. (A little too strong on the pepper, but that's a common sin of mine.) But the gravy completely failed -- what went in as a nice smooth white gravy completely broke in the pot, so I wound up with essentially lamb broth. The flour seems to have just kind of disappeared, as (mysteriously) did the milk.
So I'm mystified. I don't make rouxs often, so I presumably made a mistake, likely an elementary one, but I have no idea what. Are there traps I should be aware of when making a roux gravy to bake like this?