Preheat the oven to 425°F. Line a large sheet pan with parchment paper.
Roast the potatoes and onion: Add the potatoes and red onion to the sheet pan. Drizzle with 2 tbsp olive oil, then season with salt, pepper, and smoked paprika (if using). Toss well, then spread into an even layer with potatoes mostly cut-side down. Roast for 20–25 minutes, flipping once halfway through.
Add asparagus to the pan along with tomatoes and drizzle on more olive oil. Sprinkle some more salt and stir. Roast for 10–12 minutes, until asparagus is bright green and tender-crisp and potatoes are crisp.
Make the chimichurri: While the pan is roasting, add parsley, cilantro, oregano, garlic, lemon zest, lemon juice, maple syrup, salt, and pepper to a food processor. Pulse until finely chopped. With the machine running (or in pulses), stream in the olive oil until you have a loose, spoonable sauce. Taste and adjust—more lemon if it needs brightness, more salt if it tastes flat.
Finish and serve: Spoon chimichurri over the hot sheet pan hash and toss gently, or serve it on the side for drizzling. Taste and season with a final pinch of salt and pepper if needed. Top with more cilantro/parsley and optional microgreens.