In saucepan, melt butter over medium heat; add onion and cook, stirring, until softened but not browned, 3 to 4 minutes. Stir in garlic; reduce heat and cook, stirring frequently, until garlic is softened but not browned, 2 to 3 minutes. Add 1 cup water and salt, and bring to a boil. Add broccoli. Reduce heat and simmer, covered, until bright green and tender, 7 to 8 minutes.