In a wide, deep skillet, heat the oil over medium-high heat. Once the oil is hot, add the beef, onion, red bell pepper, green bell pepper, 1 teaspoon of the salt, and pepper. Cook, breaking apart the meat, until it is fully cooked through and the onions are translucent, about 8 minutes. Increase the heat slightly and continue to cook and stir until most of the liquid cooks away.

Stir in the garlic. Let cook, stirring constantly, until super fragrant, about 30 seconds. Add the diced tomatoes in green chilis and the tomato sauce. Let simmer for 10 minutes, stirring occasionally.

Pour 1/2 cup of the reserved pasta water back into the now-empty pot you used to cook the pasta. Add the cream cheese, Greek yogurt, cornstarch, and remaining 1/4 teaspoon salt. Whisk until smoothly combined. Stir in one-third of the cheese, continuing to whisk until it is mostly smooth. Don’t worry if the cheese doesn’t melt completely.

Add half of the green onions, the noodles, and the remaining 1 cup pasta water. Stir to combine, then carefully spoon the noodles and the liquid in the pot into an even layer in the bottom of the prepared baking dish. It will look like a mess at this point.

Spoon the beef mixture evenly over noodles. Sprinkle with the cheddar.

Bake ground beef casserole until the cheese is browned and it is bubbling around the edges, about 30 minutes. If desired, pop the pan under the broiler for a minute or two to further brown the cheese. Watch carefully. Remove from the oven, sprinkle with the remaining green onions, and let stand for 15 minutes prior to serving.