Preheat oven to 400°F and line a baking sheet with parchment paper.
Toss the plantain slices and asparagus spears with half of the avocado oil, garlic powder, and onion powder.
Spread the vegetables in a single layer on the baking sheet and roast for 18 minutes until the plantains are golden and tender.
Pat the salmon fillet dry and season both sides with the remaining garlic powder, onion powder, smoked paprika, and dried oregano.
Heat the remaining avocado oil in a cast-iron skillet over medium-high heat.
Place the salmon skin-side down in the hot skillet and sear for 5 minutes until the skin is crispy, then flip and cook for another 3 minutes.
In a small bowl, whisk together the plain kefir, lemon juice, and chopped fresh parsley to create a zesty dressing.
Plate the crispy salmon alongside the roasted plantains and asparagus, then finish by drizzling the kefir herb sauce over the fish.