Preheat oven to 350 F.

In small bowl, make a slurry by whisking 3 Tbsp flour into 3 Tbsp mushroom broth. Set aside.

In large saucepan, heat olive oil over medium and sauté onion until translucent. Add garlic, celery, mushrooms, rosemary, and 1/2 cup mushroom broth, and continue sautéing until mushrooms and celery are soft. Add lentils and 3 cups broth to mushroom mixture and bring to a simmering boil.

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.