Melt butter in a large skillet over medium heat. Add onion slices; stir to separate the slices into individual pieces. Cook until onions just begin to brown - about 5 minutes stirring occasionally. Reduce heat to medium-low and cook until onions are tender and caramelized, about 15 to 20 minutes. Remove to a bowl; set aside.
Preheat oven to 500 degrees F.
Using the same skillet, add Italian sausage and return heat to medium. Cook until sausage is browned and cooked through, about 5 to 8 minutes. Drain; set aside.
Roll out pizza dough to a 14 to 16" round {depending on your baking sheet/pizza stone size}, and place on baking sheet or pizza stone.
Bake dough for 5 mins. Carefully remove crust from oven.
Top with a thin-ish layer of marinara to within 1/2-inch of the dough edge.
Then top with sausage and almost all the onions. Sprinkle pizza evenly with cheese, and the remaining onions.
Bake for another 8 to 12 minutes until cheese is bubbly, crust edges are golden brown and the bottom is crispy. Remove from oven, slice, and eat!