Slice bell peppers in half lengthwise through stems (retain stems) and remove seeds. Place halves, cut side down, in a 12- by 17-inch baking pan. Bake in a 450º regular or convection oven until lightly browned and tender when pierced, 13 to 18 minutes.
Meanwhile, in a 10- to 12-inch frying pan over high heat, stir onions, garlic, thyme, mint, and rosemary in olive oil until onions are limp and beginning to brown, about 5 minutes.
Add ground lamb and 3 tablespoons lemon juice. Stir until lamb is browned and crumbly, 2 to 3 minutes.
Stir in broth, couscous, and remaining 2 tablespoons lemon juice. Bring to a boil, then cover and remove from heat; let stand until liquid is absorbed and couscous is tender to bite, 3 to 4 minutes. Stir in parsley; add salt to taste.
Turn pepper halves over; fill each with about 2/3 cup lamb mixture. Sprinkle feta cheese evenly over filling. Bake until cheese is slightly melted, 3 to 5 minutes. Transfer peppers to a platter.