Make the herb seasoning. Set a large (10-inch) skillet over medium heat. Lay in the garlic and chiles and roast, turning regularly, until soft and browned in spots, about 10 minutes for the chiles and 15 minutes for the garlic. (If you’re really short on time, you can soften them in a microwave: Cut a slit in each garlic clove and combine with the chiles in a microwaveable bowl. Cover with plastic wrap, poke a few holes in the top and microwave at 100% for 30 seconds.) Cool until handleable, then slip off the garlic’s papery husks. Roughly chop everything (no need to remove the chile seeds). In a blender or food processor, combine the garlic and chiles with the cilantro, parsley, olive oil and salt. Process, stopping to scrape down the sides if necessary, until nearly smooth (it should look a little like pesto) Transfer to a pint-size jar and store, covered in the refrigerator, where it will last several months.
Finish the ceviche. In a large bowl, combine the lime juice and the fish. Cover and refrigerate for 20 to 30 minutes. Stir in ½ cup of the herb seasoning and the tequila. (Cover and refrigerate the remainder of the herb mixture for another preparation.) Add cucumber, and stir to combine. To blend the flavors, cover and refrigerate for a few minutes (for best results no more than an hour). Taste and season with a little more lime juice or salt if you think necessary, gently stir in the avocado (save out a little for garnish if you want), then serve on lettuce leaf-lined plates or martini glasses.