Steam them on the stovetop until tender (about 6 to 10 minutes). If do not have a steamer basket, you can microwave them in a microwave-safe bowl with some water on high for 3 to 4 minutes.
Meanwhile, bring the chicken broth to a boil in a large nonstick pot.
Add the lemon juice, mustard, and thyme. Then, stir in the mushrooms.
Boil uncovered until the broth is reduced by half (about 5 to 8 minutes).
Add the Brussels sprouts (or other cooked vegetable).