Preheat oven to 375 degrees F and line a baking sheet with a silicone mat.
Combine cubed bread and buttermilk in a large bowl and let sit for 10 minutes.
After 10 minutes, add in onion, mushrooms, turkey, garlic, egg, salt, pepper, and sage.
Mix together, shape into 4 patties, and place onto prepared baking sheet.
Whisk together ketchup and vinegar and spread evenly over the 4 patties.
Bake in preheated oven for about 30 minutes, or until the center of the patties are no longer pink and cooked through.
While the burgers are cooking, melt coconut oil in a large non-stick skillet over medium heat.
Once melted, stir in mushrooms, salt, and pepper.
Cook, stirring occasionally for 3 minutes.
Add in vinegar and cook until mushrooms are tender, about 1-3 additional minutes.
Serve cooked meatloaf and mushrooms on One Bun Whole Wheat 100 Cal Sandwich Buns and enjoy!
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