3-Ingredient Sausage Dinner With Lentils and Fennel

Preheat oven to 400°F. Finely chop half of 1 fennel bulb. Chop 2 Tbsp. fronds, transfer to a small bowl, and set aside.
Heat 3 Tbsp. oil in a 3–4-qt. heavy saucepan over medium. Cook chopped fennel, 1/2 tsp. salt, and 1/4 tsp. pepper, stirring, until softened, about 5 minutes.
Add lentils, 1 tsp. salt, and 4 1/2 cups water, bring to a boil, then reduce heat and simmer, uncovered, until lentils are just tender but not falling apart, 15–25 minutes.
Meanwhile, cut remaining 1 1/2 fennel bulbs into 1" wedges.
Heat 1 Tbsp. oil in a large ovenproof skillet over medium-high. Cook sausages, turning occasionally, until golden brown on all sides but still undercooked, about 8 minutes. Transfer sausages to a plate.
Heat 2 Tbsp. oil in same skillet over medium-high. Cook fennel wedges and remaining 1 tsp. salt and 1/4 tsp. pepper, stirring, until golden brown on both sides, about 5 minutes. Nestle sausages among fennel wedges, then transfer skillet to oven and roast until sausages are firm and fully cooked through, about 8 minutes more.
Cut sausages diagonally into 1/2"-thick slices. Divide lentils among bowls, then top with sliced sausage, roasted fennel, a drizzle of oil, and reserved fennel fronds.