4. Step
Add the remaining 1 tablespoon oil to the pan and heat over medium heat. Add onion; cook, stirring occasionally, until soft and translucent, about 2 minutes. Add mushrooms and garlic; cook, stirring occasionally, until fragrant, about 5 minutes. Add spinach; cook, tossing often with tongs, until wilted, about 2 minutes. Stir in marinara sauce and the reserved ground beef mixture. Bring to a simmer over medium heat; simmer, undisturbed, until slightly reduced, about 5 minutes.