Note, these instructions are written assuming the standard serving size, since you have modified the number of servings, these steps may need to be modified for best results
Move an oven rack to the upper middle position, about 5" from the broiler element. Preheat oven broiler to high heat. Coat a broiler pan, or a cooling rack set over a rimmed baking sheet, with nonstick spray.
In a medium bowl, whisk together egg, almond meal, flax, mustard, 1/2 tsp. salt, and 1/4 tsp. pepper. Using a spatula or your hands, fold in beef and onion and mix until uniformly combined.
Shape the beef mixture into 4 patties, pressing down in the middle of each with your thumb to make an indentation.
Arrange on broiler pan and broil 10 to 15 minutes, turning halfway through broiling time, until a thermometer inserted in the thickest part of a burger reaches 160F.
Meanwhile, arrange the lettuce leaves on individual plates or on a serving platter.
Remove the broiler pan from the oven and let the burgers rest for 2 to 3 minutes.
Place the burgers on the lettuce leaves and serve, offering tomato slices, pickles, and extra whole-grain mustard on the side if desired.
Our recipes are created using Paleo, Primal and Gluten-free guidelines. Learn more about our diet and lifestyle at www.primalpalate.com.