In a medium bowl, toss to combine the cucumber, shallot, vinegar, sugar, 1 teaspoon salt and 1/4 teaspoon pepper; set aside.
In a large shallow dish, stir soy sauce and honey together; add chicken and turn to coat.
In a large skillet, heat oil over medium-high.
Add garlic and coriander; stir to coat.
Add chicken breasts (reserve the marinade) and cook until browned on both sides; about 3 minutes per side; browning happens a little faster than usual here because of the honey and soy; if the marinade is getting too dark, lower the heat slightly.
Add reserved marinade and 1/4 cup water to the skillet; bring to a simmer, reduce heat to medium-low and cook, covered, until cooked through; 4 to 5 minutes more per side.
Uncover the skillet, increase heat to medium-high and cook, turning chicken occasionally, until liquid is reduced and chicken is glazed; about 5 minutes.
Serve chicken drizzled with leftover glaze over rice with cucumber salad; sprinkle with cilantro, if using.
