For the Crema: 

1.  Roast the jalapeño until the outside is blackened and crisp. This can be done over a gas stove, in a dry cast iron pan, or under an oven broiler.
2.  Remove from the heat and place in a plastic ziploc bag for 10 minutes until cooled enough to touch.
3.  Peel off the skins – it should come off very easily and cleanly if you rub it off with a paper towel.
4.  Discard the stem and seeds and place the jalapeño(s) in a blender with cream, sour cream,  lime juice and zest, and salt.
5.  Blend until smooth and creamy.
6.  Add cilantro and blend until small cilantro pieces are incorporated throughout.
7.  Taste and adjust seasonings, adding more salt, pepper or lemon to your liking.
8.  Chill until ready to use.

For the Veggie Tacos

1. Slice the portabello mushrooms and asparagus. Place them on a plate and drizzle them with olive oil,  garlic powder, salt and freshly ground black pepper. Use your hands to coat the mushrooms on all sides.
2. Heat a grill or grill pan to medium-high heat. Grill the mushroom slices and asparagus until char marks form, about 3-4 minutes per side.
3.  Sprinkle with chili powder
4.  Assemble tacos with mushrooms, asparagus, cilantro and onion, sliced chilis, if desired. Drizzle with Jalapeño Cilantro Lime Crema and sprinkle Cotija to garnish.