Preheat oven to 425°F. Toss together bell peppers, 1 tablespoon oil, and 1/4 teaspoon salt in a large bowl. Place bell peppers, cut side down, on one half of a large baking sheet. In the same bowl, toss together serrano chiles, 1/4 teaspoon salt, and remaining 1 tablespoon oil. Place chiles on other half of baking sheet.
Roast in preheated oven until bell peppers and chiles are slightly charred and the skins begin to blister, about 10 minutes for chiles and 20 minutes for bell peppers. Transfer to a large bowl; cover with plastic wrap. Let stand 5 minutes. Peel and discard skins.
Heat cumin seeds in a small skillet over medium-high, stirring often, until cumin is fragrant, about 2 minutes. Transfer to a spice grinder, and finely grind.
Place bell peppers, chiles, 2 teaspoons ground cumin, garlic, Aleppo pepper, lemon juice, and paprika in a blender. Process until smooth, about 45 seconds. Season with remaining 1 1/2 teaspoons salt.