In a small bowl, whisk together the soy sauce, maple syrup (or honey), Dijon or whole grain mustard, rice vinegar, grated garlic, sesame oil, and optional lime zest. Add the chicken thighs and toss to coat. Let marinate for at least 20–30 minutes, or longer if you have time.
Meanwhile, make the cucumber salad: toss sliced cucumbers with rice vinegar, sesame oil, and sugar. Let sit for 5–10 minutes to lightly pickle. Sprinkle with green onion and sesame seeds, if using.
Remove the chicken from the marinade and let any excess drip off — this prevents burning, since the marinade contains sugar. Heat a large nonstick or cast iron skillet over medium-low heat and add a light drizzle of oil. Once hot, add the chicken and cook for 4–5 minutes per side, adjusting the heat as needed, until golden and cooked through.
Once the chicken is cooked, pour the reserved marinade into the pan. Let it simmer for 2–3 minutes until thickened and sticky, stirring to coat the chicken.
Serve the sticky sesame dijon chicken over a bowl of rice or noodles, with the cucumber salad on the side. Top with extra sesame seeds and green onions for crunch and color.