Step 1

Heat oil in a medium saucepan over medium heat. Cook onion, stirring often, until soft but not brown, about 5 minutes. Add garlic, oregano, and red pepper flakes; season with salt and pepper and cook, stirring, until fragrant, about 1 minute. Add tomato paste and 1 Tbsp. water and cook, stirring, until tomato paste coats onion and begins to darken, about 3 minutes.

Step 2

Add tomatoes to saucepan, crushing with your hands as you add them. Bring to a boil, reduce heat, and gently simmer until slightly thickened, 20–25 minutes; season with salt and pepper.

Step 3

Preheat broiler. Cover a rimmed baking sheet with foil and coat with nonstick spray.

Step 4

Using your hands or a fork, gently mix egg, onion, garlic, turkey, spinach, Parmesan, breadcrumbs, parsley, fennel seeds, lemon zest, and salt in a large bowl just until combined. Scoop out turkey mixture by the ¼-cupful and form into balls (you should have about 20). Place on prepared baking sheet, spacing 2” apart; brush with oil.

Step 5

Broil meatballs, turning often, until browned all over and cooked through, 15–18 minutes. Add to marinara sauce.

Step 6

DO AHEAD: Meatballs with marinara sauce can be made 2 weeks ahead. Let cool completely and freeze individual portions in resealable plastic bags. Run under tepid water to loosen from bag and gently reheat, covered, until meatballs are warmed through and sauce is bubbling, 15–20 minutes.