Cut the tomatoes in half and place on a baking sheet cut side up. Drizzle with olive oil and season with ½ teaspoon salt, ½ teaspoon black pepper and Italian seasoning. Roast in the oven until the tomatoes have softened, and begun to brown, about 15 to 20 minutes. Remove from the oven and set aside.
In a large bowl, mix together the ground sirloin, sausage, Worcestershire sauce, and yellow mustard until evenly incorporated. Try not to over-mix. Divide the mixture into 4 equal pieces and then shape each into a burger patty, about ¾ of an inch thick. Place on a plate or platter and season both sides with the remaining salt and pepper.
Place a griddle or cast iron skillet over medium-high heat. Once hot, crush with oil and place the patties on top. Cook for about 5 to 8 minutes on the first side, flip over and continue to cook for another 3 to 5 minutes. Cut the mozzarella into ½-inch thick slices and place on slice on top of each burger. Pop under a preheated broiler for a few minutes to melt slightly and brown.
In a small bowl, stir together the mayo and pesto until evenly combined. Brush a hot skillet or griddle with butter and place the split buns on and toast. Remove and place on a platter.
To assemble the burgers, spread each bun half with the pesto mayo and place a burger patty on top of each bottom half. Top with 2 roasted tomatoes on each and a few leaves of basil. Replace the top bun and skewer with a long toothpick to hold in place.