Heat olive oil in a large pot over medium heat. Add the diced onion and sauté for a few minutes until softened.
Season the chicken with salt and granulated garlic. Add the chicken to the pot with the onions and cook until lightly browned.
Stir in the diced tomatoes, tomato sauce, and tomato paste, and water.
Bring to a gentle boil. Stir in the cinnamon and agave, reduce heat to low, and simmer 30 minutes, stirring occasionally.
While the chicken is simmering in the sauce, in a small pan over medium heat, melt butter. Cook, swirling, it turns a golden brown and the butter smells nutty.
Stir the browned butter into the sauce. Continue simmering for 30 more minutes, until the chicken is tender.
When simmering, don’t cover the pot fully. Keep the lid slightly tilted so steam can escape and the sauce reduces properly.
Stir grated kefalotiri into the sauce just before serving.
For macaronada (optional), cook your spaghetti in salted water until just past al dente. Drain, then toss with browned butter and grated kefalotiri (add a spoon of pasta water to help coat it, if needed).
Plate the buttered, cheesy spaghetti and ladle the chicken and plenty of sauce over top so it blankets the pasta. Top it off with plenty of cheese!