Heat oil in a skillet over medium heat. Add onion; cook and stir until translucent, about 5 minutes. Add seafood stock and tomato paste. Add bok choy, garlic, gumbo file, salt, pepper, and saffron; cook and stir until well incorporated, about 5 minutes. Add tilapia. Let simmer until flavors blend and fish flakes easily with a fork, 15 to 20 minutes.
