Cook pasta shells according to package directions for al dente consistency, reserving 1/4 cup of the cooking water. Drain in a colander and set aside.

In a large pot over medium heat, melt butter then add the onion. Cook for 2 minutes, stirring frequently until softened. Add the garlic and cumin. Continue cooking for an additional 30 seconds.

Whisk in the flour, salt, and black pepper until combined. Add the chicken-style broth. Continue whisking to form a thick paste.

Reduce the heat to low then whisk in the oat milk and reserved cooking water until fully incorporated.

Remove from heat and stir in the shredded cheese and green chiles. When the cheese is melted, stir in the cooked pasta shells. Toss to coat the pasta with the cheese sauce then adjust the seasoning to taste and serve hot.