For the lobster ravioli, blend the salmon and garlic in a food processor until smooth.
Add the egg whites to the blended salmon a little at a time, blending after each addition.
Add the double cream to the salmon and egg mixture in a thin stream, blending all the time, until all of the cream has been incorporated into the mixture. Season, to taste, with salt.
Transfer the salmon mixture to a bowl, cover, and chill in the fridge until firm, about 30 minutes.
Meanwhile, for the spring vegetables, bring a large pan of salted water to the boil, then add the carrots, leeks, turnips, peas, asparagus and mushrooms and boil for three minutes. (If you don’t have a pan large enough to hold all of the vegetables, divide them among several pans.)
Drain the vegetables, then plunge them into ice-cold water to refresh. When the vegetables are cold, drain again, then set aside until needed.
When the salmon has chilled, make the ravioli. Using a small teaspoon (or a piping bag), place a marble-sized amount of the chilled salmon mixture into the centre of one of the wonton wrappers.
Repeat the process until half of the wonton wrappers and all of the salmon mixture have been used up.
Place one slice of the cooked lobster and one watercress leaf on top of each portion of salmon mixture.
One by one, brush the edges of one side of each of the remaining wonton wrappers with a little cold water. Place it, moistened-side down, on top of the prepared ravioli, pressing down at the edges to seal the two wonton wrappers together. Trim away any excess using a sharp knife.
Repeat the process with the remaining wonton wrappers to create ravioli. Cover the ravioli with clingfilm and set aside.
For the beef, preheat the oven to 180C/350F/Gas 4.
Roll the beef fillet tightly in heatproof cling film (see tip below) to form a perfect cylindrical shape, then – with the cling film still on – slice the fillet into four equal portions. Season both sides of each fillet slice with salt.
Heat an ovenproof frying pan over a medium to high heat, then add the oil. When the oil is hot, fry the fillets for 20-30 seconds on each side, or until browned on both sides.
Transfer the pan to the oven and continue to cook the beef fillets for four minutes (for medium-rare), or until cooked to your liking.
Remove the beef fillets from the pan and remove the cling film from each. Return the pan to the heat and fry the parts of the beef that were wrapped in cling film, turning regularly, until browned all the way around.
Set the beef fillets aside to rest for 5-10 minutes. Keep warm.
Add the mushrooms and lobster claws to the pan that the beef was cooked in and fry for 1-2 minutes, or until warmed through.
Meanwhile, for the spinach, heat the butter in a frying pan over a low to medium heat. When the butter has melted, add the spinach and fry, stirring constantly, until wilted. Remove from the heat and keep warm.
Bring two large pans of salted water to the boil, then reduce the heat until the water is simmering.
Add the spring vegetables to one pan and the ravioli to the other (in batches if necessary). Simmer for 4-5 minutes, or until the vegetables are cooked to your liking and the ravioli are cooked through. Drain well.
To serve, divide the spring vegetables equally among four serving bowls. Spoon over some of the fried mushrooms. Place one fillet of beef on top of each portion. Rest one lobster claw on top of each fillet. Divide the ravioli among the four dishes, placing them on top of the beef and lobster. Drizzle over some of the hot beef jus.