Preheat the oven to 350°F (175°C).
Place the 6 chicken breasts in a large stockpot and cover with water; bring to a simmer over medium heat, then cook, covered, until tender, about 18–22 minutes.
Use a slotted spoon to remove the chicken to a plate to cool slightly, reserving the cooking liquid in the stockpot.
Add the elbow noodles to the reserved cooking liquid and cook over medium heat until al dente, about 8–10 minutes, then drain the noodles in a colander, reserving a little of the liquid if desired.
Chop the cooked chicken into bite-sized pieces.
In a large mixing bowl, combine the cooked noodles, chopped chicken, grated Monterey Jack cheese, cream of mushroom soup, chicken stock or broth, chopped onion, chopped garlic, mayonnaise, and chopped pimento; stir until well combined.
Divide the mixture evenly between two 9×13-inch baking dishes and smooth the tops.
Sprinkle the crushed Ritz crackers evenly over the tops if using.
Bake in the preheated oven for about 30 minutes, until heated through and the topping is golden.
Remove from the oven and let rest a few minutes before serving.