Heat the oven to 375°F. Line a rimmed baking sheet with parchment paper. In a small bowl, stir together the breadcrumbs and ¼ cup milk. Set aside so the crumbs can soften.

Warm the olive oil in a skillet over medium heat. Add the diced onion and cook for 3 to 4 minutes, stirring, until softened. Add half of the minced garlic and the spinach. Cook until the spinach wilts and any liquid in the pan evaporates. Transfer the mixture to a bowl, stir in the Parmesan, and let it cool.

In a large bowl, combine the ground turkey, chicken sausage, egg, softened breadcrumb mixture, Italian seasoning, ½ teaspoon salt, and ¼ teaspoon black pepper. Mix gently just until combined.

Place the meat mixture on the prepared baking sheet and shape it into a rectangle about 9 by 12 inches. Spread the cooled spinach mixture down the center, leaving a border around the edges. Fold the long sides of the meat mixture over the filling and pinch the seams closed. Turn the loaf seam-side down.

Mix the ketchup and tomato paste in a small bowl. Spread the glaze over the top of the meatloaf. Bake until the center reaches 165°F, about 50 to 60 minutes. Let the meatloaf rest for 10 minutes before slicing.

While the meatloaf bakes, put the potatoes in a large pot and cover them with cold water. Bring to a boil, then simmer until the potatoes are tender when pierced with a fork, about 15 to 20 minutes. Drain well and return them to the pot.

Add the butter, warm milk, remaining minced garlic, remaining ¼ teaspoon salt, and remaining ¼ teaspoon black pepper to the potatoes. Mash until creamy. Serve the sliced spinach-stuffed meatloaf with the garlic mashed potatoes.