Step 1

Butter an 8x8" or 9x9" baking dish with unsalted butter.

Step 2

Whisk 7 large eggs, 1¾ cups whole milk, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a medium bowl until well combined. Season with freshly ground black pepper and set aside.

Step 3

Heat 2 Tbsp. unsalted butter and 2 tsp. vegetable oil in a large skillet over medium until butter is melted. Cook ½ tsp. cumin seeds, stirring constantly, until fragrant and starting to pop, about 1 minute. Reduce heat to medium-low and add 2 large leeks, white and pale green parts only, thinly sliced; season with salt. Cook, stirring occasionally, until leeks are very soft and starting to brown, 8–10 minutes. Increase heat to medium and add 1 medium red bell pepper, cut into ½" pieces. Cook, stirring often, until pepper is beginning to soften, about 3 minutes. Add 4 garlic cloves, finely grated, and one 1" piece ginger, peeled, finely grated, and cook, stirring constantly, until fragrant, about 30 seconds. Add 1 tsp. garam masala, 1 tsp. ground coriander, 1 tsp. ground cumin, ½ tsp. ground turmeric, and ½ tsp. mild chile powder and cook, stirring constantly, until fragrant, about 1 minute.

Step 4

Scrape mixture into a large bowl and let cool slightly. Stir in ½ medium bunch cilantro, leaves and tender stems coarsely chopped, ¼ medium red onion, chopped, 9 oz. sourdough bread, preferably day-old, cut or torn into 1" pieces, and 1½ cups coarsely grated sharp cheddar; season with salt and pepper. Transfer to prepared baking dish and spread out into an even layer. Pour reserved egg mixture over; gently press down on strata to help bread absorb egg mixture. Scatter remaining ½ cup coarsely grated sharp cheddar over. Cover strata and chill at least 1 hour and up to 12 hours.

Step 5

Preheat oven to 350°. Bake strata until top is golden brown and a knife inserted into center comes out clean, 45–50 minutes. Let cool slightly, then cut into squares. Serve topped with more cilantro and Cilantro Chutney, if desired.Do Ahead: Strata can be baked 3 days ahead. Let cool completely; cover and chill. Reheat in a 375° oven until hot, 20–25 minutes.