Combine cilantro, garlic, lime juice, red pepper flakes, ¼ tsp. each of the no-salt substitute and black pepper in a blender. Blend, gradually pouring in all but 1 Tbsp. of the olive oil, until mostly smooth.
Transfer mixture to a bowl; gently toss with diced avocado.
Heat a grill pan or cast-iron skillet over high heat.
Rub tuna steaks with reserved 1 Tbsp. oil; sprinkle with remaining ½ tsp. no-salt substitute and ¼ tsp. black pepper.
Cook the fish for 3-4 minutes each side.
To serve, slice tuna steaks and spoon avocado salsa verde overtop.