- In a saucepan over medium heat, bring the stock to a gentle simmer and maintain over low heat.
- In a large, heavy saucepan over medium heat, melt 2 tablespoons of the butter with the olive oil. Add the onion, celery and garlic and sauté until softened, about 4 minutes.
- Add the sea bass and sauté until firm and opaque, 4 to 5 minutes. Using a slotted spoon, transfer the sea bass mixture to a bowl and set aside.
- Add the rice to the same pan and stir until each grain is well coated with oil and translucent with a white dot in the center, about 3 minutes. Add the wine and stir until it is completely absorbed.
- Stir in the fennel slices. Add the simmering stock a ladleful at a time, stirring frequently after each addition. Wait until the stock is almost completely absorbed (but the rice is never dry on top) before adding the next ladleful. Reserve 1/4 cup stock to add at the end.
- When the rice is tender to the bite but slightly firm in the center and looks creamy, after about 20 minutes, gently stir in the sea bass mixture, the reserved 1/4 cup stock and the remaining 1 tablespoon butter. Cook to heat through, about 1 minute.
- Season with salt and pepper and spoon into warmed shallow bowls. Garnish with fennel fronds and serve immediately.