Combine soy sauce, mirin, sugar, and minced garlic to make the marinade.
Marinate the chicken breasts in the mixture for at least 30 minutes.
Heat a pan over medium-high heat and cook the chicken on both sides until nicely browned.
Pour the remaining marinade over the chicken and simmer until the sauce reduces and thickens.
Once the chicken is fully cooked (internal temperature of 165°F), remove from heat and let it rest for a few minutes before slicing. Serve with the reduced sauce.
Notes
Always marinate your chicken for at least 30 minutes, longer if you have the time, to maximize flavor.
When cooking the chicken, ensure your pan is hot before adding the chicken to get a nice sear.
Don't rush the reduction of the sauce; a low and slow simmer will produce a thicker, more flavorful sauce.
Use a meat thermometer to make sure the chicken is cooked to the correct temperature (165°F).
Be sure to baste your chicken with the sauce while it cooks to make it flavorful and juicy.