01 - Preheat the oven to 425°F. Line a baking sheet with parchment paper.
02 - Arrange the bell pepper quarters and red onion slices on the prepared baking sheet. Drizzle with 1 tablespoon of olive oil and season with a pinch of salt. Roast for 20 to 25 minutes, turning once halfway through, until the vegetables are tender and lightly charred.
03 - While the vegetables roast, rinse the millet under cold running water. Combine the millet and vegetable broth in a medium saucepan and bring to a boil. Reduce the heat to low, cover, and simmer for 15 minutes until all liquid is absorbed. Remove from heat and let stand, covered, for 5 minutes. Fluff with a fork.
04 - In a small bowl, whisk together the remaining olive oil, lemon juice, Dijon mustard, minced garlic, sea salt, and black pepper until well emulsified.
05 - Slice the roasted bell peppers into strips. In a large serving bowl, combine the cooked millet, roasted peppers, roasted onions, cherry tomatoes, and baby spinach.
06 - Pour the prepared dressing over the salad and toss gently until all ingredients are evenly coated.
07 - Fold in the chopped parsley, torn basil, and crumbled feta cheese if using. Toss lightly and serve warm or at room temperature.