Preheat your oven to 360ºF.
Saute the onions with the olive oil on medium high heat until transparent and lightly browned.
Steam the broccoli using a steamer for 5-7 minutes. (see alternatives and more explanation above).
Add the steamed broccoli, sauteed onions, eggs, flour, sour cream, salt, and pepper. Mix gently.
Spread the butter into a casserole dish or spray with nonstick spray. Add the broccoli mix and press it down gently.
Bake for 50 minutes, then sprinkle over the mozzarella and bake for 10 more minutes. Slice and enjoy!