Trim any skin from the tuna with a very sharp knife, then cut the tuna into 1/4-inch cubes and place them in a bowl.
In a medium bowl, whisk together the soy sauce (or alternative), lime juice, grapeseed oil, sesame oil, honey, minced ginger, minced garlic, and red pepper flakes until combined.
Pour the marinade over the tuna cubes and toss to coat evenly.
Cover and refrigerate the tuna to marinate for at least 45–50 minutes.
While the tuna marinates, slice the Persian cucumber 1/8-inch thick (use a mandolin if available) and roll the slices; cut the watermelon into 1/2-inch cubes and thinly slice the shallot.
To serve, divide the marinated tuna among four plates, arrange watermelon cubes and rolled cucumber slices around the tuna, and garnish with sliced shallot and cilantro or microgreens. Taste and add salt if desired just before serving.