Heat oil in a skillet over medium-high heat. Add onion; saute until translucent, about 5 minutes.
Combine the almond flour mixture, onion, ground beef, 1/4 cup ketchup, eggs, horseradish, salt, mustard, and pepper in a bowl. Mix well and pour into a 10x5-inch loaf pan.
Mix the remaining ketchup, vinegar, and sugar substitute together in a small bowl. Spread topping on top of loaf.
Bake in the preheated oven until browned, 45 minutes to 1 hour. An instant-read thermometer inserted into the meatloaf should read about 155 degrees F (68 degrees C).
