Spray a casserole dish with cooking spray. Put the lentils in the dish and set it aside. Preheat the oven to 350 degrees.
Slice the sausage into 1/2" coins. Heat a medium skillet over high heat. Add the sausage slices and cook until lightly browned on one side, 3 - 4 minutes.
Turn the sausage and cook another 3 - 4 minutes. When the second side is browned, remove the sausage from the pan add to the lentils in the casserole.
Add the tablespoon of olive oil to the pan and swirl to coat the bottom. Add the onions, mushrooms, and garlic and cook for 5 minutes or so stirring often. Add some of the chicken stock if things seem dry.
Once the veggies are soft and fragrant, add them along with any drippings to the casserole. Add the rosemary and Italian seasoning.
Mix all the ingredients together and cover with foil. Bake in the oven for 30 - 35 minutes or until things are bubbly.
Layer the tomato slices on top of the casserole. Sprinkle with the cheese and drizzle with a little more olive oil.
Return to the oven uncovered and bake another few minutes until the tomatoes begin to soften and the cheese melts.
Turn the oven to broil and let the casserole go 3 - 4 minutes until it is golden. Remove from the oven and let set 10 minutes before serving.