Quarter potato lengthwise. Cut into ½" pieces. Place on prepared baking sheet and toss with 1 tsp. olive oil, ½ tsp. salt, and a pinch of pepper.
Spread into a single layer. Roast 20 minutes.
Potatoes will finish cooking in a later step.
While potatoes roast, halve and peel onion. Cut halves into ¼" slices.
Mince garlic and combine with 1 tsp. olive oil.
Core tomato and cut into ¼" slices.
Make Pesto-Mayo and Form Patties
Combine half the pesto (reserve remaining for mozzarella) and mayonnaise in a mixing bowl.
Combine remaining pesto and mozzarella in another mixing bowl.
Form ground beef into four equal-sized patties (about ½" thick and 4" in diameter). Divide mozzarella-pesto mix between the middle of two burger patties. Place remaining patties on cheese-topped patties and seal edges by pinching lightly. A tight seal helps cheese stay in burger while cooking.
Cook Onions and Toast Buns
Place a medium non-stick pan over medium heat and add 2 tsp. olive oil. Add onions to hot pan and stir occasionally until tender, 5-7 minutes.
Add balsamic vinegar and stir occasionally, 1-2 minutes.
Remove from burner. Transfer onions to a plate.
Wipe pan clean and reserve.
While onions cook, halve buns, if necessary. Place cut side down directly on oven rack and toast until lightly browned, 2-3 minutes.
FInish Potatoes and Cook Burgers
After potatoes have roasted 20 minutes, drizzle with garlic-oil mixture. Roast until garlic is fragrant and potatoes are lightly browned, 5 minutes.
While potatoes roast, return pan used to cook onions to medium heat and add 2 tsp. olive oil. Add burgers to hot pan and cook until burgers reach a minimum internal temperature of 160 degrees, 5-6 minutes per side.
Cheese may leak from burgers. Don't worry, they're still delicious!
Remove from burner.
Finish the Dish
Plate dish as pictured on front of card, garnishing with sauce, onion, and tomato. Bon appétit!