Combine cilantro, olive oil, juice from 1 lime, 1 garlic clove and ¾ teaspoon sea salt in a blender and combine until smooth, stopping to push cilantro down into the blades as necessary. Set aside.
Heat 1 tablespoon oil in a large skillet over medium heat. Add sliced onion and fennel and cook 5 minutes, until it turns golden brown but still has some crunch. Add 2 minced garlic cloves, red pepper flakes and a big pinch of sea salt; cook for 1 minute, stirring frequently. Add broth or water and chard. Reduce heat to medium-low, cover pan and cook for 5 minutes. Uncover skillet, raise heat back to medium-high, and cook until the water/broth is evaporated, stirring occasionally. Season with sea salt, to taste.
Divide chard and fennel between tortillas and top with cilantro sauce and sliced radishes.