How to Make It
Preheat oven to 350°F. Heat oil in a large skillet over medium-high. Add spinach to skillet; cook, stirring constantly, until wilted, about 2 minutes. Transfer spinach to a bowl. Pulse bread slices in bowl of a food processor until finely chopped, about 15 seconds. Set aside.
Combine pork, sirloin, reserved breadcrumbs, pecorino Romano, Parmesan, eggs, garlic, parsley, milk, black pepper, red pepper, and 1 1/2 teaspoons of the salt in a large bowl. Wrap cooked spinach in paper towels, and squeeze out excess liquid; finely chop. Add chopped spinach to meat mixture. Gently combine with hands or a fork until incorporated, but not overworked.
Form 9 miniature loaves (about 4 ounces each) from meat mixture, and place on a baking sheet lined with parchment paper. Bake until a thermometer inserted in thickest portion registers 155°F, 30 to 35 minutes. (Temperature will rise to 160°F while resting).
Meanwhile, combine marinara sauce, balsamic vinegar, brown sugar, and remaining 1/4 teaspoon salt in a small bowl. During the last 10 minutes of cooking, brush meatloaves with marinara mixture every few minutes until meatloaves are cooked through and sauce is thick and sticky, about 3 times. Remove from oven, and serve immediately or store in refrigerator up to 3 days.