Preheat the oven to 190C/Gas 5. Cut the potatoes into chunky, even-sized chips and place in a large pan of lightly salted boiling water. Boil for 6-7 minutes, then drain thoroughly. Return to the pan, replace the lid, and shake to roughen the edges of the chips (this helps them to crisp up during baking).
Line a baking sheet with nonstick baking parchment and spread out the chips. Spray with oil and sprinkle with sea salt. Place in the oven and bake for 15-20 minutes, or until the chips are crisp and golden.
About 5 minutes before the chips are ready, gently beat the eggs together with the herbs and water. Season well.
Spray a medium, non-stick frying pan with a thin coating of oil and place over a high heat. When hot, pour in the egg mixture and cook for 1-2 minutes. As the egg begins to set around the edges, add the cooked mushrooms and use a spatula to push the egg towards the centre. When the entire egg mixture is set, cook for another minute, then loosen the edges with a spatula and fold the omelette in half. Serve with the chips and grilled tomatoes.