Preheat the oven to 375 degrees. In a medium bowl, combine the panko, melted butter, and garlic. Set aside.
In a large oven safe skillet, melt the butter over medium heat. Add the onions and sauté for about 5 minutes. While the onions cook, bring a large pot of salted water to a boil.
Reduce the heat to medium low. Add the garlic and sauté for 2-3 minutes.
Stir in the half and half, salt, and pepper.
Add the cream cheese. Stir continuously until the cream cheese is mostly melted. Whisk the mixture to incorporate any small clumps. The half and half should not be boiling. This will take about 8 minutes.
Reduce the heat to the lowest setting. Stir in the parmesan cheese. It does not have to be completely melted.
Boil the tortellini in the prepared pot of water for one minute less than the package directs. I usually boil mine for 2 minutes. Reserve about 1 cup of the pasta water. Drain the tortellini and add the cooked tortellini to the alfredo sauce.
Gently, stir in about 1/2 cup of the reserved pasta water and about 1 cup of the cheese until just combined. Do not overmix. Add more pasta water if needed. The cheese does not have to melt completely. Smooth the pasta into an even layer and top with the remaining mozzarella.
Sprinkle the panko mixture on top of the cheese.
Bake for 8-10 minutes. Move the skillet to the top oven rack and broil for 2-3 minutes or until golden brown, watching closely so the panko does not burn. Serve immediately.