Cook the brown rice according to package instructions until tender.
Thinly slice the Korean cucumbers into rounds and place them in a medium mixing bowl.
In a small jar, whisk together the rice vinegar, half of the tamari, half of the gochugaru, and the minced garlic clove.
Pour the dressing over the cucumbers, toss well to coat, and let them marinate in the refrigerator while you prepare the shrimp.
Pat the shrimp dry with a paper towel and season both sides with sea salt, black pepper, and the remaining gochugaru.
Heat the sesame oil in a large skillet over medium-high heat until shimmering.
Add the shrimp to the skillet in a single layer and sear for 2 to 3 minutes per side until they are opaque and slightly charred.
Drizzle the remaining tamari over the shrimp in the last 30 seconds of cooking to glaze them.
Divide the warm brown rice and chilled cucumber salad between two plates or bowls.
Top with the seared shrimp and garnish with sliced green onions and toasted sesame seeds.