While stirring continuously, slowly add slurry to lentil mixture and return to a simmering boil; allow mixture to thicken and reduce. Add salt and pepper, to taste.
Transfer lentil and mushroom mixture to 9 x 13 inch casserole dish and top evenly with a layer of mashed potatoes.
Place casserole dish in preheated oven with large baking sheet on the oven rack below it; bake, uncovered, for 20 to 25 minutes.
Recipe Notes
Per serving: 231 calories; 9 g protein; 4 g total fat (1 g sat. fat); 41 g total carbohydrates (2 g sugars, 5 g fiber); 355 mg sodium