Preheat oven to 400 degrees.
Place cauliflower on a sheet pan; drizzle with olive oil and toss to coat. Season with sea salt and pepper and roast for 20-25 minutes, until golden brown and tender.
Place onions in a large glass or ceramic bowl. Add sugar, sea salt and peppercorns to a small saucepan. Add vinegar and stir until sugar and salt are dissolved. Bring mixture to a boil over medium-high heat. Once boiling, pour vinegar over sliced onion, until fully submerged. Let it come to room temperature.
Turn oven to broil. Place tortillas on a large baking sheet, then in oven for 30 seconds. Remove, flip tortillas and repeat on other side. Wrap tortillas in a clean dish towel to keep warm.
Divide cauliflower between tortillas. Drizzle with chimichurri and top with pickled onion and sliced avocado.