Melt 1 tablespoon of butter in a large straight-sided skillet or Dutch oven over medium-high heat. Add the chicken, season with salt and pepper, and cook until golden brown but not fully cooked through, about 4 to 6 minutes total. Remove the chicken to a plate.

Reduce the heat to medium-low and add the olive oil and remaining tablespoon of butter. Once melted, increase the heat to medium and add the chopped onion. Season with salt and pepper and sauté for about 4 minutes, stirring occasionally.

Add half of the diced tomatoes and cook for 1 minute. Stir in the garlic, Italian seasoning, and crushed red pepper flakes. Cook until fragrant, about 30 seconds.

Sprinkle the flour over the vegetables and cook, stirring constantly, for 1 to 2 minutes.

Slowly pour in the chicken broth and half-and-half, whisking constantly until smooth. Bring the mixture to a simmer.

Return the browned chicken to the skillet and add the linguine. Bring to a slow boil, then immediately reduce to a simmer.

Cover and cook for 10 to 15 minutes, stirring once or twice during cooking so the pasta does not stick, until the pasta is just al dente.

Reduce the heat to low and stir in the parmesan cheese until melted. Add the remaining tomatoes and adjust seasoning with salt and pepper.

Remove from the heat and stir in the basil. Dollop with ricotta and gently stir to combine. Garnish with more basil, parmesan, and crushed red pepper flakes if desired. Serve warm.