Preheat your oven to 425°F. Prepare a rimmed baking sheet with a wire rack and coat it with cooking spray to ensure the tempeh doesn’t stick.
Combine ¼ cup water, 4 tablespoons rice vinegar, and 1 tablespoon each tamari and brown sugar in a medium bowl. Toss with napa cabbage and radishes, then set aside to pickle as you prep the other components.
In a small saucepan, whisk together the remaining 1 tablespoon vinegar, 3 tablespoons each tamari and brown sugar, gochujang, garlic, and ginger. Bring to a gentle simmer over medium-high heat, stirring occasionally for 2 minutes.
Mix the remaining ¼ cup water with the cornstarch in a small bowl. Gradually whisk this slurry into the sauce until it thickens, about 1 minute.
Transfer half the sauce to a medium bowl and toss with the tempeh pieces to coat evenly. Arrange the tempeh on the prepared wire rack and bake for 15 minutes, or until the glaze becomes tacky.
Remove the tempeh from the oven and toss with the reserved sauce for an extra layer of flavor.
To assemble, divide the cooked brown rice among four bowls. Use a slotted spoon to transfer the pickled vegetables to each bowl, and drizzle with a little of the pickling liquid if desired. Top with the glazed tempeh and sprinkle with fresh cilantro for a burst of color and freshness.