In a large skillet, heat oil over medium-high. Pat fish dry and season both sides with salt and pepper. Add fillet to skillet, flesh side down, and cook until fish is golden brown and releases easily from skillet, about 5 minutes. Gently flip and cook until opaque throughout, about 2 minutes. Transfer to a plate. Add escarole, orange zest, and olives to skillet. Cook, stirring constantly, until escarole wilts, about 2 minutes. Add orange juice and toss to coat. Serve alongside trout.
Farmed rainbow trout is a good source of omega-3s. Escarole is high in folate and vitamin A.