Place a large sauté pan over medium heat. Add the 1 pound Italian sausage and cook, breaking it up with a spoon or spatula as needed, until browned and cooked through (about 6–8 minutes).

Transfer the cooked sausage to a plate or bowl and set aside. Carefully pour off and discard any excess fat from the pan, leaving the browned bits.

Return the pan to medium heat and add the 1 tablespoon olive oil. When the oil is hot and shimmering, add the 1 cup diced sweet onion and 1 cup diced red bell pepper. Season with salt and pepper to taste. Sauté until the vegetables have released their water, the water has evaporated, and they are soft and beginning to turn golden (about 6–8 minutes).

Add the 4 cloves garlic (minced) and 1 teaspoon dried oregano to the vegetables; sauté for about 30 seconds, until fragrant (avoid browning the garlic).

Stir in the 28-ounce can crushed tomatoes, then return the cooked sausage to the pan and stir to combine. Bring the mixture to a gentle simmer.

Reduce the heat to low and simmer gently for at least 20 minutes, stirring occasionally, to let the flavors combine.

Taste and adjust salt and pepper as needed before serving.