Using microwave or vegetable steamer, steam broccoli until tender.
In a large, nonstick skillet, melt butter over medium high heat. Add in onions and garlic and sauté for about 3-4 minutes or until onions begin to soften and become translucent.
Transfer cooked onions and garlic to a blender. Add in the steamed broccoli, feta cheese, olive oil , lemon juice, water, and salt & pepper. Puree until smooth. Add additional water if sauce is too thick.