Preheat a grill pan over moderate heat until hot.
While the pan heats up, trim the sea bass fillet and cut it into four even squares or portions. Pat dry with paper towels and season with salt and pepper.
When the grill pan is ready, drizzle each fillet with 1 Tablespoon oil. Brush the grill pan with oil and place the fillets in it; let cook undisturbed until the flesh starts to release from the pan, about 2 minutes; using tongs, carefully turn the fillets 90° clockwise and leave to cook until almost opaque, about 2 minutes. Flip and cook until the flesh is firm to the touch and opaque on all sides, 1-2 minutes. Remove to a plate and loosely cover with aluminum foil to keep warm.
Steam the asparagus in a covered steaming basket set over a half-filled saucepan of simmering water until just tender, about 2 minutes.
Divide the asparagus, sea bass, and tomatoes between plates. Serve straight away for best results.
