Preheat oven to 400°F (200°C). Pat salmon dry. In an oven-safe skillet over medium heat, warm 1 tbsp olive oil; sauté onion and bell pepper 5–6 minutes. Add garlic; cook 1 minute.
Stir in lentils, oregano, red pepper flakes, salt, and black pepper. Add broth; bring to a simmer.
Reduce heat to low, cover, and cook 20–25 minutes at a gentle simmer, stirring once or twice. If liquid reduces too quickly, stir in ½–1 cup hot water or broth to keep lentils just submerged; cook until tender with slight bite.
Uncover; stir in cherry tomatoes, olives, and capers. The mixture should be thick, not watery.
Nestle salmon, skin-side down, into the ragoût; drizzle remaining 1 tbsp olive oil over the fillets.
Transfer skillet to the oven; roast, uncovered, 15–18 minutes (for ~1-inch/2.5 cm thick fillets). The salmon is done when it flakes easily or reaches 145°F (63°C). For thinner fillets, begin checking at 12 minutes.
Remove from oven. Squeeze the juice of half the lemon over the dish; garnish with parsley and the zest of the whole lemon. Serve immediately, spooning lentil ragoût onto plates and topping with a salmon fillet.