Step 1: Preheat oven to 400 degrees.

Step 2: In a medium bowl, mix ground beef, 2 tbsp mustard, and a generous pinch of salt using your hands until just combined. Roll into 16 small balls (about 1½ inches wide) and place on a rimmed baking sheet. Gently press each ball down to slightly flatten into mini patties.

Step 3: Bake for 10 minutes, then remove from the oven. Place a small square of cheese on each burger. Return to the oven for 1-2 minutes until cheese is melted and burgers are fully cooked.

Step 4: While burgers bake, toast hamburger buns until lightly golden. Cut each bun into quarters to create 8 mini buns. Spread extra mustard on each mini bun if desired. Prepare the toppings – slice tomatoes and chop spinach.

Step 5: On a toothpick, layer in this order: Bottom bun (mustard side up), spinach, tomato slice, mini cheeseburger, top bun (mustard side down). Gently press down on each mini burger to help them stand upright.