Heat avocado oil in a large skillet over medium heat.
Add diced onion, jalapeño, and chopped garlic to the skillet. Cook for 5-7 minutes until vegetables are softened and fragrant.
Stir in taco seasoning and cayenne pepper (if using). Cook for 1 minute until spices are aromatic.
Add black beans to the skillet and cook for 5-8 minutes, stirring occasionally, until mixture has thickened.
Using a fork or potato masher, lightly mash some of the beans to create a chunky texture while leaving some beans whole.
Remove from heat and stir in lime juice and sea salt to taste.
Warm tortillas according to package directions or over an open flame.
Assemble tacos by filling each tortilla with the black bean mixture and desired toppings.