Linguine With Rustic "Meatballs"

INGREDIENTS

• 1 tablespoon cornstarch
• 1 large red onion, chopped
• 1 cup sweet (red) vermouth
• 1 teaspoon ground cumin
• 2 celery ribs, finely chopped
• 2 medium carrots, finely chopped
• 2 garlic cloves, finely chopped
• 1 tablespoon fresh lemon juice
• 4 tablespoons extra-virgin olive oil, divided
• 1/3 cup celery leaves, coarsely chopped
• Accompaniment: grated Parmigiano-Reggiano
• 1 pound thin linguine or spaghetti
• Rounded 1/8 teaspoon hot red-pepper flakes
• 1 3/4 cups reduced-sodium chicken broth
• 1 1/4 pounds meatloaf mix (ground beef, pork, and veal; preferably not mixed together)