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In a large oven-safe skillet, melt the butter over medium heat. Add the diced onion and cook about 3 minutes, until it starts to soften. Add the tomato paste, garlic and red pepper flakes and cook, stirring constantly, about 30 seconds.
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Stir in the canned tomatoes and then salt and pepper to taste. Reduce heat to a simmer and let it cook while you prep the meatballs.
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Preheat the oven to 425 degrees. Line a rimmed cookie sheet with foil and set aside.
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In a food processor, pulse the onion until it is finely diced. Add the remaining ingredients and pulse until well-combined. Roll the mix into meatballs about the size of golf balls (I used a cookie scoop) and place on the baking sheet. Cook for 15 minutes - it's okay if they aren't cooked quite all the way through.
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Take the sauce off the heat and stir in the cream or half and half. Add the meatballs to the pan of sauce, sprinkle the two cheeses over the top and put in the oven to bake until the cheese on top is bubbling and the meatballs are cooked through, 10-15 minutes.
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Garnish with the fresh basil, if you're using it. Serve warm over pasta or with french or garlic bread.