In a small bowl, whisk together the gochujang, tamari, rice vinegar, and minced garlic to create the marinade.
Slice the chicken breast into thin strips and toss with half of the gochujang sauce; let marinate for 10 minutes.
Heat a non-stick skillet over medium-high heat with half of the sesame oil. Sauté the mushrooms, carrots, and bean sprouts individually for 2-3 minutes each until tender-crisp, seasoning lightly with sea salt.
Lightly steam the spinach in the same pan with a splash of water until wilted, then set aside.
Add the marinated chicken to the skillet and sear for 5-6 minutes until fully cooked and slightly caramelized.
In a separate small pan, fry the egg with the remaining sesame oil until the whites are set but the yolk remains runny.
Assemble the bowl by placing the warm brown rice at the base, then arrange the chicken and vegetables in distinct sections on top.
Place the fried egg in the center, garnish with the cucumber slices, and drizzle with the remaining gochujang sauce.