Big Baked Falafel Cake With Cucumber Salad

Name: Big Baked Falafel Cake With Cucumber Salad
Cuisine: Middle Eastern
Ingredients: 1 large red onion; 3 Tbsp. plus 1 tsp. fresh lemon juice; ¼ tsp. sugar; 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt, plus more; ¾ cup (packed) cilantro leaves, divided; ¾ cup (packed) parsley leaves, divided; 2 Tbsp. cornstarch or potato starch; 1 tsp. ground cumin; ½ tsp. baking powder; ½ tsp. ground coriander; ¼ tsp. cayenne; Freshly ground black pepper; 2 garlic cloves, finely grated; 1 large egg, beaten; 2 15.5-oz. cans chickpeas, rinsed; 2 Tbsp. extra-virgin olive oil, plus more for drizzling; 1 cup plain whole-milk yogurt; 3 Persian cucumbers, thinly sliced; 2 radishes, trimmed, thinly sliced