Preheat the oven to 425 degrees. Line a rimmed cookie sheet with parchment paper.
Place the sliced carrots, mushrooms, and red onion on the sheet, and drizzle with olive oil. Bake at 425 degrees for 25-30 minutes, stirring occasionally.
Once the vegetables go in the oven, prepare the brown rice according to package instructions.
About 5 or 10 minutes before the vegetables and rice are done cooking, make the pesto sauce. Place the spinach, garlic, salt, lemon juice, and olive oil in a food processor and blend until smooth and combined. Taste and add more salt or pepper if needed.
When the rice and vegetables are done, divide among two plates. Top with some of the pesto. (You may have extra pesto left over). Enjoy!