Brown pancetta in a large pot over medium-high heat, stirring often, about 8 minutes. Transfer to paper towels with a slotted spoon, leaving fat in pot.
Meanwhile, bring broth and 1 cup water to a simmer in a small saucepan, or microwave in a glass measuring cup until hot.
Reduce heat under large pot to medium. Add enough oil to fat to make 3 tbsp. Stir in green onions, fennel, rice, salt, and 1/2 tsp. pepper; cook, stirring, until rice is opaque, about 3 minutes. Stir in broth, 1/2 cup at a time, and cook, stirring constantly until almost all the broth is absorbed before adding more, until rice is tender to the bite, about 20 minutes; the texture should be fairly soupy. Remove from heat.
Melt butter in a large frying pan over medium-high heat until just starting to brown. Add fava greens and cook, turning occasionally with tongs, until bright green, wilted, and tender, 2 to 3 minutes.
Fold fava greens and beans, 1/2 cup cheese, and the pancetta into risotto. Stir in about 1/4 cup more hot water to loosen the texture. Spoon into bowls, grind more pepper on top, and serve with more cheese if you like.
*Find at farmers' markets.
How to Double-Shell Favas: Makes 2 cups 30 minutes Tear open 3 lbs. fava pods*. Add beans from pods to a pot of boiling salted water and return to a boil. Cook just until beans are tender beneath skin (slit to check), 2 minutes. Drain; rinse with cold water. Peel leathery skin by tearing beans open at rounded ends and popping out beans. *Find at well-stocked grocery stores and farmers' markets. Make ahead: All but the final peeling, up to 1 day, chilled.