Heat oil and butter in a 6-quart heavy pot over medium heat until foam subsides
2
Add fennel, onion, 3/4 teaspoon salt, and 1/8 teaspoon pepper and cook, stirring occasionally, until softened and lightly browned, 7 to 8 minutes.
3
Add leafy greens (but not spinach) and water to pot and simmer, covered, until greens are tender, about 30 minutes. Stir in spinach and cook, uncovered, just until wilted, about 1 minute.