Melt butter over medium heat in skillet. Sauté onion and chicken until chicken is no longer pink, then add garlic and cook for 1 minute more.
Sprinkle flour over onions and stir to combine. Cook for 1 minute, then gradually whisk in milk, water, and bouillon. Cook, stirring constantly, until sauce begins to simmer. Stir in salt and pepper.
Continue stirring until sauce thickens, about 5-8 minutes.
Serve over rice, then let everyone add their desired toppings.
Recipe by I Wish I Was a Muffin at https://iwishiwasamuffin.com/hawaiian-haystacks-with-homemade-chicken-gravy/