Preheat the oven to 425 degrees F. Spray a 13-by-9-inch baking pan with cooking spray.
Cook the pasta in boiling salted water according to the package directions. Drain and reserve.
Meanwhile, grate the cauliflower on the large holes of a box grater, or chop it into chunks and pulse to rice-size pieces in a food processor.
Melt the butter in a large pot over medium heat. Add the onions and cook, stirring often, until softened, about 5 minutes. Add the grated cauliflower and 1/2 teaspoon salt and cook, stirring occasionally, until crisp-tender, another 5 minutes. Add the flour; cook for 1 minute, stirring.
Gradually add the milk while stirring constantly, then increase the heat and bring to a boil. Reduce the heat to medium and simmer, whisking occasionally, until slightly thickened, about 3 minutes.
Take off the heat and stir in the French onion dip, Cheddar, Colby Jack, cayenne and 1 teaspoon salt until melted. Add the cooked pasta and stir to coat. Transfer to the prepared baking dish.
Top the pasta with the crushed potato chips. Bake until lightly browned, 8 to 10 minutes (be careful, the potato chips can burn easily).