Prepare a sheet pan with a silpat or foil. Salt and pepper salmon on both sides, place salmon on pan, skin side down. Put salmon into cold oven. Turn oven to 400 degrees. Remove after approximately 20 - 22 minutes. You want the salmon to be slightly firm, but with a bit of give when you pull it out of the oven. Remember that there will be a degree of carryover cooking when you pull it out. You ultimately want the salmon to still be a tiny bit rare inside - the same way that you might cook a steak to medium.
While the salmon is cooking, sautée the shallots over medium heat in butter and olive oil in a sauce pan.
Once the shallots have softened, add the sherry and cook until it has reduced by almost half.
Add the cream, and reduce heat to medium low. Add mustard, diced tomato and Worcestershire sauce, and simmer until it thickens and reduces by about a quarter.
When it has reduced to your liking, taste it, and add salt, pepper, and lemon zest.
When salmon has been removed from the oven, sautée shallots and garlic in butter and olive oil in a sautée pan over medium heat.
When they have softened, add arugula and sautée until it just begins to wilt. Remove from heat.
To serve, place a salmon fillet over a bed of arugula, top with sauce, and garnish with fresh tomatoes.
