1-1/2 pounds grass-fed ground beef
1/2 cup chopped onion
1 can of tomato paste
2 TBSP pure maple syrup*
2 TBSP apple cider vinegar*
1 TBSP stone-ground mustard
Salt and pepper to taste
In a large skillet, cook beef and onion. (I typically allow the meat to be a little pink since it will further cook in the crock pot.) Transfer to a 3-qt. crock pot.
Stir in the tomato paste, vinegar, maple syrup, mustard, salt, and pepper. Cover and cook on low for 3-4 hours or until flavors are combined.
Spoon mixture over a gluten-free bun or use a lettuce wrap.
*You may need to adjust the maple syrup and vinegar to your liking. If you want more tang, add more vinegar. If you want something sweeter, add more maple syrup.