Heat the oven to 375°F.

Bring a large pot of heavily salted water to a boil over medium-high heat. Add 1 pound dried spaghetti to the boiling water and cook according to package directions for 1 minute less than al dente. Meanwhile, make the sauce.

Heat 1 tablespoon sun-dried tomato oil in a Dutch oven or large high-sided skillet over medium-high heat until shimmering. Add 1 diced medium yellow onion, 1 cup julienne-cut sun-dried tomatoes, 3 minced garlic cloves, 1 ½ teaspoons kosher salt, and ¼ teaspoon red pepper flakes if using. Cook, stirring occasionally, until the onion is softened, about 4 minutes.

Add 1 tablespoon tomato paste and cook, stirring occasionally, until darkened in color, about 1 minute.

Sprinkle with 2 tablespoons all-purpose flour and cook for 1 minute to remove the raw flour taste.

Stir in 3 cups low-sodium chicken broth, 1 cup heavy cream, and 1 ounce of the finely grated Parmesan cheese. Bring to a simmer.

Reduce the heat to maintain a gentle simmer and cook until the sauce is slightly thickened, 2 to 4 minutes.

When the spaghetti is ready, drain, then add to the sauce and toss until combined. Taste and season with more kosher salt as needed.

Transfer to a 9x9-inch square (at least 2 inches high) or 9x13-inch baking dish and arrange into an even layer.

Sprinkle evenly with the remaining 1 ounce finely grated Parmesan cheese.

Bake until the edges are bubbling and the cheese is melted, 15 to 17 minutes.

Let cool for 5 minutes. Garnish with more Parmesan cheese and torn fresh basil leaves if desired.