Add the mushrooms, garlic, onion, olive oil, and salt to a skillet

Cook the mushrooms mixture over medium high heat for around 10 minutes, until the mushrooms evaporate their liquid and cook down

In a large bowl, mix together 1 cup of the cheddar cheese, the cream of mushroom soup, sour cream, broccoli, and the mushroom onion mixture

Grease a small casserole dish and pour in the broccoli casserole. Spread it out evenly

Top the casserole with the remaining 1 cup of sharp cheddar cheese

Add the crushed ritz crackers over the top

Bake the broccoli casserole at 350 degrees for 25 - 35 minutes, until golden browned and bubbly