Heat a skillet over medium heat. Warm each tortilla on each side. Cover the tortillas to keep warm. (See notes.)
In the same skillet used to heat the tortillas, add the olive oil. When warm, add the garlic and saute for about 30 seconds.
Next add the sliced mushrooms to the skillet. Saute, stirring occasionally, until most of the liquid from the mushrooms has evaporated and the mushrooms are soft. Season to taste with salt and pepper.
Meanwhile, prepare the Chimichurri sauce by adding the parsley, oregano, garlic, red pepper flakes, red wine vinegar, and salt and pepper to the bowl of a food processor. Pulse, scraping down the sides of the bowl as needed.
Stream the olive oil through the funnel in the top while the food processor is running. Taste and adjust the seasonings, if needed.
Build the tacos by adding some of the mushrooms to each tortilla then topping with Chimichurri. Garnish with avocado slices, if desired, and enjoy.
Nutrition Facts do not include the optional avocado slices.Warming tortillas:To keep your tortillas warm for up to an hour, I recommend this inexpensive insulated Tortilla Warmer.Success tip:Do not blend the sauce until it's completely smooth. You should still be able to see small pieces of herbs in the Chimichurri.Storing leftovers:Any leftover sauce will last refrigerated for 1-2 days. Mix well before serving since the olive oil will solidify.Sauteed mushrooms can be covered and refrigerated for a few days as well. Reheat in a microwave-safe container or in a skillet over low heat.