Heat 1 Tablespoon oil in a large saucepan set over medium heat.
Add the pancetta and sauté until golden, stirring frequently, about 5 minutes.
Add the scallion, garlic, and a pinch of salt; sauté until the scallions have softened, 2-3 minutes. Stir in the lentils and stock.
Bring to a boil over high heat; once boiling, reduce to a simmer and cook until the lentils are tender, skimming away any scum that rises to the surface, 25-30 minutes.
When ready, stir in the lemon juice and some salt and pepper to taste. Cover and keep warm off the heat.
Preheat a large skillet over moderate heat. Pat the catfish fillets dry with paper towels; season both sides with salt and pepper.
When the skillet is ready, add the remaining 2 Tablespoons oil. Carefully position the catfish fillets in the skillet, skin-side down, and pan-fry until the skin is golden underneath and the flesh is starting to turn opaque, about 3 minutes. Flip the fillets and add the butter to the skillet; cook until the flesh is opaque, basting with the melted butter, 2-3 minutes.
Reheat the lentils if necessary. Divide the lentils and catfish between plates; garnish with lemon wedges and thyme.
