Step 1
Place a rack in middle of oven and preheat to 425°. Combine ½ tsp. saffron (15–20 threads) and 1 Tbsp. warm water in a large bowl; set aside.
Step 2
Meanwhile, place about ½ cup thinly sliced medium onion in a medium bowl of ice water and set aside to soak. Finely chop 1¼ cups (lightly packed) mixed tender herb leaves with tender stems (such as parsley, dill, cilantro, and/or mint) and set aside.
Step 3
Heat 5 Tbsp. extra-virgin olive oil in a large skillet over medium. Add 2 tsp. ground coriander, 2 tsp. ground cumin, 2 tsp. vegetable or chicken bouillon paste, ½ tsp. freshly ground pepper, ¼ tsp. Diamond Crystal or Morton kosher salt, and remaining onions and cook, stirring often, until onions are deep brown and crisp in spots, 13–17 minutes.
Step 4
Add one 15-oz. can chickpeas, rinsed, to pan; increase heat to medium-high. Cook, stirring occasionally, until slightly crisped, 6–8 minutes. Remove from heat; mix in ½ cup dried tart cherries or unsweetened dried cranberries and reserved chopped herbs. Season with more salt and pepper if needed.
Step 5
Cook 2 cups basmati rice, rinsed, in a medium saucepan of boiling generously salted water until just al dente, about 5 minutes, then drain.
Step 6
Add 2 large egg yolks, ¼ cup mayonnaise, ¾ cup whole-milk Greek yogurt, and ¼ tsp. Diamond Crystal or Morton kosher salt to reserved bowl with saffron; whisk until smooth. Fold in rice.
Step 7
Pour 3 Tbsp. extra-virgin olive oil into an 8×8″ glass baking dish and spread around to coat bottom and sides. Add 2 cups rice mixture and press with a flat-bottomed measuring cup or a spatula to compress. Mound onion mixture over, leaving a ½” border on all sides, and flatten slightly. Spoon remaining rice mixture on top and lightly press down to compress. Cover baking dish with foil.
Step 8
Bake tachin until bottom and sides are deep golden, 55–65 minutes. Let cool 10 minutes, then remove foil and run a paring knife around sides to loosen. Place a platter or large plate upside down over baking dish and hold together as you quickly turn over; remove baking dish.
Step 9
Whisk remaining ½ cup whole-milk Greek yogurt and 2 Tbsp. water in a small bowl (it should be thin enough to drizzle; add more water if needed). Season with salt and pepper.
Step 10
Drain reserved onion. Toss onion, a pinch of kosher salt, remaining 1¼ cups (lightly packed) mixed tender herb leaves with tender stems, and remaining 1 Tbsp. extra-virgin olive oil in same bowl to combine. Mound salad on top of tachin; serve with yogurt sauce.
