Boil water in large pot.
Add spaghetti pasta to boiling water.
Cook pasta for 8-10 minutes until tender.
Drain pasta in colander completely.
Heat olive oil in medium skillet.
Add diced chicken to hot skillet.
Cook chicken for 5 minutes until heated.
Remove chicken and set aside temporarily.
Add butter to same skillet.
Sauté onion until soft and translucent.
Add minced garlic and cook 1 minute.
Pour heavy cream into skillet slowly.
Add chicken broth and stir well.
Return cooked chicken to skillet mixture.
Add Monterey Jack cheese gradually.
Stir until cheese melts completely smooth.
Add cooked spaghetti to cheese sauce.
Toss everything together until well coated.
Season with salt and pepper.
Serve hot in individual bowls immediately.