Preheat the oven to 400°F. Peel and small dice the carrot. Finely chop the celery.
Place a small saucepan over medium heat and add the butter, diced carrots, and chopped celery. Cook until fragrant, about 2 to 3 minutes. Add the millet, vegetable broth concentrate, and 1½ cups water. Bring to a boil, reduce heat, cover, and simmer until all of the water is absorbed and the grains are tender, 15 to 20 minutes.
Peel and mince 2 cloves garlic. Mince the parsley (leaves and stems). Zest and halve the lemon. Add the garlic, minced parsley, lemon zest, juice from half the lemon, chimichurri spice, 2 tbsp olive oil, and ½ tsp salt to a large bowl. Mix to combine. Drain the tofu and cut into 1 inch cubes. Add the cubed tofu to the herb marinade and toss well to coat.
Place the wooden skewers on a baking sheet with a few cups of water to soak. Peel and chop the red onion into large pieces. Chop the zucchini into 1 inch cubes. Take a wooden skewer and alternately add pieces of the red onion, zucchini, and marinated tofu. You will get 4 to 5 full skewers. Reserve any remaining herb marinade for Step 6.
Dry the baking sheet. Place the tofu kebabs on it and sprinkle with salt. Roast the skewers in the oven until the vegetables are tender and browned in places, about 15 to 20 minutes.
Cut the remaining lemon half into wedges. Fluff the millet pilaf with a fork and season with salt and pepper. Divide the millet pilaf between large plates and top with the herb marinated tofu kebabs. Drizzle with any remaining herb marinade and serve with lemon wedges.