Bring water to a boil in a large saucepan; add cauliflower, lemon slices, and garlic. Cover saucepan and steam cauliflower until tender, 5 to 10 minutes. Drain water; remove and discard lemon slices and garlic. Transfer cauliflower to a serving bowl.
Melt butter in the same saucepan over medium-low heat; add salt and lemon juice and cook until butter sauce is smooth, 1 to 2 minutes. Pour butter sauce over cauliflower and top with egg.