Preheat the oven to 450 degrees F with the rack set in the upper third of the oven.
Roast chiles over a gas flame, on a grill, or under the broiler, allowing the skins to blacken and char. Peel the skin off the chiles. Cut off stems, and remove seeds and membranes, and finely chop the chiles.
Place the chopped chiles in a medium bowl, and mix in the chipotle paste and salt. Once those are thoroughly combined, mix in the chopped green onions, reserving some for garnishing, and the shredded cheese.
Place the three tortillas on a large baking sheet, and divide chile-cheese mixture evenly over the top of the tortillas. Bake loaded tortillas until the cheese is golden and bubbly, about 8-10 minutes. Carefully remove from the oven and set on plates. Top each tostada with shredded lettuce, diced avocado, and reserved green onion garnish. Devour!
For Printing: How to Roast Chiles: https://lifecurrentsblog.com/cooking-basics-%E2%80%93-roast-a-chili-pepper/ Make your Own Chipotle Paste: https://lifecurrentsblog.com/chipotle-paste/