Shell the peas and set aside. Bring a small pot of water up to a boil. Add the peas and cook until they turn bright green and rise to the surface, about 2-3 minutes. Drain and rinse the peas with cold water and transfer to a large food processor.
Add the watercress, garlic, walnuts, cheese, salt and lemon juice to the food processor and blitz until a thick paste forms.
Set the food processor on low and slowly stream in the olive oil. Blitz until the oil is well-integrated and the mixture is silky. Transfer the pesto to a bowl and set aside until ready to use.
For the Mac & Cheese
Bring a large pot of water up to a boil. Cook the pasta according to the package's directions.
While the pasta is cooking, melt the butter in a large, deep skillet. Whisk in the flour to form a roux. Cook until the roux takes on a slightly warm, nutty smell, about 2 minutes.
Whisk in the wine and slowly whisk in the milk. Stir in the salt and the Dijon mustard. Cook the sauce until thickened and bubbles form on the surface, about 5 minutes. Reduce the heat to a simmer and stir in the cheese. Stir until the cheese has melted and is well-integrated.
Take the sauce off of the heat and stir half of the Sweet Pea Watercress Pesto into the sauce. Reserve the other half for future use.
Drain and rinse the pasta and add it to the sauce. Toss to coat. Divide the pasta amongst 4 to 6 bowls. Drape one or two pieces of smoked salmon on top of the each dish and garnish with a sprinkling of pea shoots. Serve immediately.