Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Reserve 1/2 cup of pasta water, then drain the pasta and set aside.

While the pasta is cooking, heat a large skillet over medium-high heat. Add the sliced andouille sausage and cook for 5-7 minutes, stirring occasionally, until browned and slightly crispy. Remove the sausage from the skillet with a slotted spoon and set aside, leaving any rendered fat in the pan.

If needed, add 1 tablespoon of olive oil to the skillet. Add the sliced red bell pepper, green bell pepper, and yellow onion. Sauté for 7-10 minutes, stirring frequently, until the vegetables are tender-crisp and slightly charred. Add the minced garlic and fajita seasoning, cook for another 1 minute until fragrant.

Push the vegetables to one side of the skillet. Add the butter to the empty side and let it melt. Sprinkle in the all-purpose flour and whisk constantly for 1-2 minutes to create a roux. Gradually whisk in the chicken broth until smooth, then stir in the heavy cream.

Bring the sauce to a gentle simmer, stirring frequently, until it begins to thicken (about 3-5 minutes). Reduce the heat to low. Stir in the Monterey Jack (or Pepper Jack) and Parmesan cheeses until melted and smooth. If using, stir in the chili powder. Season with salt and black pepper to taste.

Add the cooked pasta, browned andouille sausage, and the sautéed fajita vegetables into the creamy sauce. Toss everything together until the pasta and ingredients are well coated. If the sauce is too thick, add a splash or two of the reserved pasta water until it reaches your desired consistency.

Serve hot, garnished with fresh chopped cilantro and a squeeze of fresh lime juice, if desired.