Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 425 degrees. Wash and dry produce.

Drain and rinse lentils; thoroughly pat dry with paper towels. Halve, peel, and cut onion into ½-inch wedges. Zest and quarter lemon (for 4, zest one lemon and quarter both).

Toss lentils and onion on a baking sheet with a drizzle of olive oil, garlic powder, salt, and pepper. (For 4 servings, toss lentils and onion together in a medium bowl. Divide between two sheets and roast on top and middle racks.)

Roast on top rack until lentils are browned and crispy and onion is browned and tender, 20-25 minutes.

Meanwhile, in a small bowl, combine yogurt, half the chimichurri, and as much lemon zest as you like.

When lentils and onion have 5 minutes remaining, massage grain blend in package to separate grains. Partially open package; microwave until warmed through, 1½-2 minutes.

In a large bowl, toss together spinach, grain blend, remaining chimichurri, juice from one lemon wedge (two wedges for 4 servings), salt, and pepper.

Divide grain salad between shallow bowls. Top with roasted lentils and onion. Drizzle with creamy chimichurri and serve with remaining lemon wedges on the side.