Steam or microwave (about 3 minutes on HIGH) chopped broccoli and stems until tender but still bright green. Squeeze lemon over broccoli.Melt butter in skillet. Add onions, garlic, celery and mushrooms and saute until softened. Stir in flour and cook about one minute to take away the raw flour taste. Whisk in milk and continue whisking until slightly thickened. Season with salt and pepper. Add 1½ cup cheese and stir to melt.[Microwave directions for Step 2] Add butter, onions, celery and mushrooms to 2 quart batter bowl. Cover and microwave on HIGH for 5 minutes. Add flour to vegetables and stir. Whisk in milk and cook uncovered on HIGH for 6-7 minutes. Whisk after 4 minutes and once a minute thereafter to even out the cooking. Season with salt and pepper. Add 1½ cup cheese and stir to melt. Sauce will thicken more as it sits.Combine broccoli, rice, and cheese sauce. Turn into 1½ quart greased casserole dish. Sprinkle with remaining ½ cup cheese. Bake at 350 degrees F until bubbly.