• Heat the 1 tablespoon olive oil in a large skillet over medium-high heat. Add the 1 large sweet onion (very finely diced), 2 celery stalks (very finely diced), 1 large red bell pepper (very finely diced), 1 medium zucchini (very finely diced), 1 teaspoon fine sea salt, and ½ teaspoon freshly ground black pepper.
  
• Cook for 8-10 minutes, stirring occasionally, until the vegetables are very soft and most importantly, the liquid released by the zucchini has evaporated. Add the 4-6 garlic cloves (minced) and cook for another minute.
  
• Remove from the heat and allow the vegetable mixture to cool for about 10 minutes.
• In a large mixing bowl, whisk together the 2 large eggs, ¾ cup panko breadcrumbs, ¼ cup whole milk, 1 tablespoon Dijon mustard, 1½ teaspoons smoked paprika, 1 teaspoon dried oregano, and 1 teaspoon dried parsley. Let the mixture sit for about 3 minutes so the breadcrumbs can absorb some of the liquid.
• Add the 3 pounds ground turkey and cooled sautéed vegetables. Using clean hands, gently mix everything together just until evenly incorporated.
  
Don't compact or overwork the meat mixture. This is one of the most important steps for a tender meatloaf.
• Place two long strips of parchment paper into the slow cooker in a cross pattern, leaving enough overhang to act as handles later.
• Shape the meat mixture into a compact oval loaf approximately 8-9 inches long. Place it in the center of the slow cooker.
  
• In a small bowl, whisk together the 1/3 cup ketchup, 2 tablespoon tomato paste, 1 tablespoon Dijon mustard, 1 tablespoon maple syrup or honey, 1 teaspoon apple cider vinegar, and ½ teaspoon smoked paprika.
• Spread about half of the glaze evenly over the meatloaf.
• Cover and cook on LOW for 6-7 hours, or until the center of the meatloaf reaches 160°F.
I prefer LOW rather than HIGH for this recipe because the gentler cooking produces a more tender meatloaf.
• During the final 20-30 minutes of cooking, spread the remaining glaze over the top.
• Once cooked, turn off the slow cooker and let the meatloaf rest uncovered for 10-15 minutes. Use the parchment handles to carefully lift it from the slow cooker.
  

Optional caramelized finish:

• For a more traditional sticky meatloaf crust, transfer the cooked meatloaf to a baking sheet. Brush with any remaining glaze and broil for 2-4 minutes, watching closely, until bubbling and lightly caramelized.
• Rest for another 5 minutes before slicing. Serve with mashed potatoes and a simple green vegetable or salad.