Preheat your oven to 375˚F.

Bring a medium pot of salted water to a boil. Cook the shells until almost al dente, about 8 minutes. Drain pasta in a colander and return the pot to the burner.

While the pasta is cooking, heat the olive oil in a medium skillet over medium to medium-high. Cook jalapeños, onion, salt and pepper, stirring occasionally for about 2 minutes. Stir in garlic and cook for an additional minute. Remove from heat and set aside.

Add 1 Tbsp butter to the pot you used for the pasta. Once melted and foaming, whisk in flour and cook, whisking constantly, until golden brown, about 2 minutes. Slowly whisk in the milk, cooking until it thickens slightly.

Stir in 3/4 of the grated cheese until melted and sauce is thick. Stir in jalapeño onion mixture, shredded chicken and cooked pasta.

Pour the mixture into a casserole dish (about 2 to 3 quarts). Top with remaining shredded cheese. Bake for 30 minutes, until cheese is browned and bubbling.

Remove from heat and serve immediately.