Pre-heat your oven to 425 degrees. Line a large baking sheet with parchment or foil.
Place the cauliflower florets in a large mixing bowl. Drizzle with 1½ tablespoons olive oil, and add the cumin, paprika, and a generous pinch of salt and pepper. Toss to coat. Arrange the cauliflower in an even layer on the prepared baking sheet. Roast for 20-25 minutes, turning once halfway through the cooking time, until the florets are tender and nicely browned in spots.
While the cauliflower is roasting, get going on the chimichurri. Place the bell pepper and tomatoes in a food processor and pulse a few times until they’re broken down. Add the basil, cilantro, jalapeño, ginger, garlic, and balsamic pulse a few more times until you have a chunky sauce. Drizzle in the olive oil and process for 5-10 seconds until incorporated. Transfer to a bowl and season with salt and fresh ground pepper to taste. Briefly set aside while you cook your tuna.
Pat the tuna steak(s) dry. Season both sides generously with salt and fresh ground pepper. Heat the remaining tablespoon of oil in a large non-stick skillet over medium-high heat. When hot, add the tuna and cook for about 90 seconds per side. We’re just searing the tuna, peeps. It will be raw and cool in the center. (If raw-ish tuna isn’t your thing, cook your tuna for an extra minute or two per side.)
Transfer the tuna to a cutting board. Slice the steaks thinly across the grain.
Time to assemble your bowls! Add ½ cup of grains to each of 4 bowls. Divide the roasted cauliflower among the bowls, and fan the sliced tuna alongside the cauliflower. Top with plenty of red chimichurri.