Baba Ghanoush Bagel Sandwich

Name: Baba Ghanoush Bagel Sandwich
Ingredients: 1 large eggplant (1 to 1 1/2 lb.), halved lengthwise; 2 cloves garlic, grated or finely chopped; 1/4 c. tahini; 2 tbsp. exrtra-virgin olive oil; 2 tbsp. fresh lemon juice; 1/4 tsp. crushed red pepper flakes (optional); Kosher salt; Freshly ground black pepper; 1 small red onion, sliced (about 1 c.); 1 tbsp. fresh lemon juice; 1 tsp. sumac; 1/4 tsp. kosher salt; 4 sesame bagels, halved, toasted; Za'atar, for sprinkling; 3 mini cucumbers, thinly sliced; 4 radishes, thinly sliced; 2 c. arugula; Extra-virgin olive oil, for drizzling