In a food processor, combine chickpeas, parsley, garlic, cumin, baking powder, and salt. Pulse until the mixture looks like coarse meal with some texture remaining - not a smooth paste. You should still see tiny bits of chickpea. Transfer to a bowl and refrigerate for 15 minutes to firm up; this helps the patties hold their shape.

Preheat oven to 425°F. Line a large baking sheet with parchment. Scoop out 2 tablespoons of mixture and form into a compact patty about 2 inches wide and 0.5 inch thick - you will get about 16. Brush both sides lightly with 2 tablespoons olive oil. Bake for 22-25 minutes, flipping once halfway through, until deep golden and crisp at the edges.

While falafel bakes, whisk together tahini, lemon juice, minced garlic, salt, and warm water in a small bowl. It will seize up at first - keep whisking and add more water 1 teaspoon at a time until you have a smooth, pourable sauce the consistency of heavy cream. Taste and adjust salt or lemon as needed.

Divide warm grains among four bowls. Top each with 4 falafel patties, then arrange cucumber, tomatoes, and red onion around them. Drizzle generously with tahini sauce and serve the rest on the side. Eat while the falafel is still warm and crisp.