Heat the oven to 375°F. Line a rimmed baking sheet with parchment paper.
Warm the olive oil in a large skillet over medium heat. Add the mushrooms and onion. Cook for 8 to 10 minutes, stirring occasionally, until the mushrooms release their moisture and the mixture looks dry. Stir in the garlic and cook for 30 seconds. Set the mixture aside to cool.
In a large mixing bowl, lightly mash the cooked lentils with a fork, leaving some whole for texture. Add the ground turkey, egg, ground flaxseed, cooled mushroom mixture, tomato paste, Worcestershire sauce, thyme, salt, and pepper. Mix gently until combined.
Shape the mixture into a loaf on the prepared baking sheet. Spread the ketchup over the top.
Bake for 50 to 60 minutes, until the center reaches 165°F on an instant-read thermometer. Rest the meatloaf for 10 minutes before slicing.