Preheat the oven to 375F and line a baking sheet with parchment.
In a medium pot over medium heat, add the oil, scallions, and ¼ teaspoon salt and cook, stirring occasionally until softened, about 3 minutes. Stir in the brown rice, remaining ½ teaspoon salt, ginger, soy sauce, and water. Bring to a boil, then reduce heat to medium-low and simmer rapidly, covered, for 30 minutes or until cooked through (there will still be water in there!).
Remove the lid and simmer rapidly until it’s a thick soupy consistency (you want it to be a little bit brothy, kind of like risotto), about 5-10 minutes. Remove from heat and stir in the rice vinegar.
In a medium bowl, add the tamari or soy sauce and miso and use a fork to mash and mix the miso into the soy sauce so that it dissolves. Stir in the maple syrup, rice vinegar, garlic and red pepper flakes. Add the tempeh and toss gently to coat. Let it marinate for at least 20 minutes.
Transfer the marinated tempeh to the baking sheet, reserving as much marinade as you can, and spread in an even layer so they aren’t touching. Bake for 10-15 minutes, or until caramelized.
Serve the warm rice with tempeh and cucumbers. Drizzle the tempeh with the remaining marinade and sprinkle the reserved green scallions all over (or any other fresh herb). Squeeze the lime over the cucumbers and sprinkle everything with sesame seeds (if using).