Boil pasta according to package directions, adding 1 tablespoon salt to the pasta water (optional). Drain and set aside.
Meanwhile, melt the butter in a medium saucepan on medium heat. Add minced garlic and sauté until fragrant, about 30 seconds to 1 minute.
Add the pesto and tuna to the saucepan, breaking up the tuna with the spatula. Heat until warm, about 2 minutes.
When the pesto-tuna mixture is warmed through, slowly add the heavy cream, stirring as you pour. Heat until warm, about 2 minutes.
Remove the saucepan from the heat and stir in the onion powder, salt, pepper, and parmesan until the cheese melts. Next, toss in the cherry tomatoes.
Combine cooked and drained pasta with the sauce in a large pot, stirring to coat the pasta evenly. Add more salt or pepper to taste. Serve warm.