Preheat the oven to 205°C and line a baking sheet with parchment paper.
Rinse the quinoa a few times in a fine mesh strainer. Bring a medium pot of water to a boil. Add the quinoa and cook until tender, 10 to 15 minutes. Strain the quinoa and rinse under cold water, then set aside to drain and dry out.
Meanwhile, combine the spinach, cilantro and garlic in a food processor and pulse until finely chopped. Transfer one-third of the mixture to a medium bowl along with the ground beef and turkey, milk, breadcrumbs, 2 teaspoons of the Worcestershire, the chile powder, egg, 1/2 teaspoon salt and some black pepper. Combine the mixture well with your hands.
Whisk together the remaining 2 teaspoons Worcestershire with 2 teaspoons of the vinegar. Divide and pat the meat mixture into four even 1-inch-thick football shapes. Arrange on the prepared baking sheet. Top each with 3 slices of tomato and drizzle with some of the Worcestershire-vinegar sauce. Bake until the meatloaf reaches an internal temperature of 75°C, 18 to 20 minutes. Drizzle with more Worcestershire-vinegar sauce and top each with cheese and continue to bake until melted, about 2 minutes more. Blot the bottom of each meatloaf on a paper towel to remove excess moisture.
Meanwhile, combine the quinoa, the remaining spinach mixture, the remaining 2 teaspoons vinegar and 1/2 teaspoon salt in a medium pot and stir, over medium-low heat, until warmed through, about 3 minutes. Divide the meatloaves, quinoa and watermelon slices among four dinner plates.