How to Make It
Preheat broiler and set a rack 3 in. from heating element. Line a large rimmed baking pan with foil and put tomatoes, onion, and garlic in it. Broil the vegetables, turning as needed, until browned in spots all over, 15 to 20 minutes; transfer to a bowl as done. Let cool.
Meanwhile, wipe dried chiles clean with a damp cloth. Pull out and discard seeds and stems from cascabels (break chiles open a bit if needed). Turn on fan over stove. Heat olive oil in a large frying pan over medium-high heat, add cascabel and arbol chiles, and cook, turning often with a slotted spoon, until slightly softened and darkened in spots, 1 to 2 minutes.
Reserving oil in pan, transfer chiles to a small, deep bowl and pour 2 cups boiling water on top. Let stand until chiles are softened, about 20 minutes. Discard 1 cup liquid.
Whirl chiles and remaining liquid with chipotle in a food processor until very smooth. Cut tomatoes and onion into chunks. Peel garlic. Add vegetables to chile purée and pulse until nearly smooth.
Reheat oil in pan over medium-high heat. Add chile mixture and bring to a simmer, stirring. Cook, stirring, for about 3 minutes to blend flavors.
Pour salsa into a bowl and let cool. Stir in lemon juice and season to taste with salt.
*Buy in your supermarket's international foods aisle or at a Latino market.
Make ahead: Chill airtight up to 1 week.
Note: Nutritional analysis is per 1/4-cup serving.