Preheat the oven to 375 degrees and lightly grease a 12-cup muffin tin.
In a large bowl, combine the ground turkey, chopped spinach, breadcrumbs, egg, diced onion, minced garlic, ketchup, salt, black pepper, and thyme. Mix until evenly blended.
Divide the mixture evenly among the muffin cups, pressing it down gently to fill each one.
Bake the muffins for 25 minutes until they reach an internal temperature of 165 degrees.
While the muffins bake, place the cubed sweet potatoes in a pot and cover with water. Bring to a boil and cook for 15 minutes until tender.
Drain the sweet potatoes, then return them to the pot. Add the butter and milk, then mash until smooth. Season with a pinch of salt if needed.
Serve the meatloaf muffins warm with a scoop of sweet potato mash on the side.