To make this dip, start by adding the olive oil to a 10-12 inch cast iron skillet or oven-safe pan. Heat the pan, on the stove, over medium-high heat.
Once the pan is hot, dice the onion and sauté it for about 3-4 minutes. Then, once the onion is almost translucent, add the sausage to the pan and use a spatula to break it down into small pieces.
Once the sausage is fully cooked through, remove the pan from the heat and set it aside.
Next, preheat your oven to 375F.
Then, soften the cream cheese. You can do this one of two ways: either by leaving the cream cheese out until it reaches room temperature, or by transferring it into a microwave-safe bowl and microwaving it for 20 seconds.
Add the softened cream cheese to a bowl alongside the sour cream, mozzarella cheese, parmesan cheese, mashed garlic, Italian seasoning, sea salt, tomatoes, and the cooked sausage and onion mixture. Use a spoon to mix everything together well, until it's fully incorporated.
Then, transfer the mixture back into the skillet and use the back of the spoon (or a spatula) to spread the mixture out evenly across the pan.
Then, place the skillet into the oven and allow the dip to bake for 20-22 minutes, or until the skillet is heated all the way through and the cheese on top is melted and bubbly.
Once the dip is done, remove it from the oven. Serve it immediately alongside your favorite sliced baguette, chips, or crackers, and enjoy!