Test Kitchen's Best Spaghetti and Meatballs

Heat a straight-sided skillet over medium, swirl in oil; add onion and 1 teaspoon salt. Cook, stirring occasionally, until onion is soft and translucent, 7 to 9 minutes. Stir in garlic and red pepper flakes; cook 30 seconds. Transfer one-third of onion mixture to a bowl and reserve for meatballs.

Add tomatoes to skillet; bring to a boil. Reduce heat and simmer, stirring occasionally, until reduced and thickened slightly, about 20 minutes.

While sauce simmers, heat a large skillet over medium-high and swirl in oil. Add mushrooms and cook, stirring occasionally, until soft and most of moisture has evaporated, 8 to 10 minutes. Transfer to bowl with reserved onion mixture; let cool slightly.

Preheat broiler with rack 4 inches below heating element. Stir breadcrumbs and milk into mushroom mixture until milk is absorbed. Add cheese, parsley, oregano, egg, 1 teaspoon salt, and 1/4 teaspoon pepper. Add beef and gently fold with your hands until evenly combined

Use an ice cream scoop, or roll heaping tablespoons between dampened palms, to form thirty 1 1/2-inch balls. Transfer to a rimmed baking sheet brushed with oil. Broil, undisturbed, until browned on top, about 5 minutes.

Transfer meatballs to skillet with red sauce. Simmer, stirring gently, until just cooked through, about 5 minutes.

Meanwhile, boil pasta in a large pot of generously salted water until al dente, about 2 minutes less than per package instructions. Drain pasta and return to pot.

Toss pasta with 1 1/2 cups sauce. Serve, topped with meatballs, more sauce, cheese, red-pepper flakes, and parsley.