Slice ocean trout diagonally into 8 thick, even steaks. Heat 20gm butter in a large non-stick frying pan over high heat, add half the ocean trout and cook, turning once, until golden and almost cooked through (30 seconds to 1 minute each side). Set aside and keep warm; repeat with remaining trout.
Blanch baby beans until just tender, adding snow peas and podded peas during the last minute of cooking (2-2½ minutes). Drain and set aside.
Add shallots to frying pan and stir until just tender (2-3 minutes), then add thyme and stir to combine. Add lemon juice and remaining butter and whisk to combine, then season to taste. Spoon sauce over warm trout and vegetables, scatter with watercress and serve with sourdough.