Preheat the oven to 350 degrees F. Bring a large pot of salted water to a boil. Butter a 9-by-13-inch baking dish.
Cook the spaghetti according to the package instructions. Drain and then rinse. Shake off any excess water and transfer to the prepared baking dish.
Melt the butter in a large saucepan over medium heat. Add the cumin, scallion whites and garlic and cook, stirring, until soft, about 4 minutes. Sprinkle in the flour and cook, stirring, until completely absorbed, about 1 minute. Add the milk, 1 teaspoon salt and a few grinds of pepper and cook, stirring, until the sauce is heated through, steaming and thickened slightly, about 5 minutes. Remove from the heat and add small handfuls of half of each cheese, stirring to melt after each addition.
Stir in the turkey, pickled jalapenos, brine and tomatoes. Adjust the seasoning with salt and pepper.
Pour the sauce over the cooked spaghetti, using a wooden spoon or rubber spatula to evenly distribute the turkey into the pasta. Sprinkle the top with the remaining cheeses. Bake, uncovered, until melted and bubbly, 25 to 30 minutes. Sprinkle with the scallion greens.
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