In a blender or food processor, combine the oats, flour, parsley, lemon zest and some freshly ground black pepper. Blend until the oats are coarsely ground. Add the olive oil and blend until the mixture just comes together.
Arrange the fish in a single layer, skin side down, in a shallow 4 cup ovenproof dish that fits the fillets snugly. Pour over the stock and lemon juice, and top with the crumble topping, spreading it evenly over the fish.
Bake for 15 minutes, until the topping is well-browned and the fish is just cooked through. Serve hot.
For more crunch, leave some of the rolled oats whole, or add some nuts to the mixture.