Bring water and salt to a boil in a saucepan. Cook spaghetti at a boil until cooked through but firm to the bite, about 12 minutes; drain.

Melt butter in a skillet over medium heat. Stir flour into the melted butter; add chicken stew, garlic, onion, and oregano. Cook and stir the chicken stew mixture until thickened, 5 to 7 minutes.

Melt 2 1/2 teaspoons cheese into the chicken mixture. Add spaghetti and toss to combine. Top with remaining cheese to serve.