Preheat the oven to 180°C.
Melt butter in a saucepan and cook shallots for 3 minutes until soft.
Pour in champagne and reduce by half, then add stock and reduce by half again.
Stir in double cream and simmer until thickened.
Stir in the lemon zest and half of the Gruyère cheese, then set aside.
Blanch kale in boiling water for 2 minutes, then drain and pat dry.
Layer sauce, kale, salmon, basil, and pasta sheets in an oven proof dish.
Repeat layers and finish with sauce and remaining cheese.
Bake for 25 minutes until golden and bubbling.
Whisk olive oil, vinegar, and mustard together to create the dressing.
Toss the rocket and watercress with the dressing and serve alongside the lasagne.