Step 1Spray the bottom of a large skillet with nonstick spray, or use a nonstick skillet. Fill the skillet 3/4 full with water and bring to a boil.
Step 2Meanwhile, whisk together the Dijon mustard, Sherry vinegar and lemon juice until blended. Stir in the shallot. Slowly whisk in the olive and canola oils until the dressing thickens and is emulsified. Stir in salt and pepper to taste. Set aside. Toss the watercress leaves with the dressing and divide among 4 plates.
Step 3Reduce the boiling water to a simmer. To poach the eggs, work with 2 at a time. Crack the first egg into a small bowl, then carefully pour into the water; repeat with the second egg. Cook until the whites of the eggs are set and cooked through, about 3 to 4 minutes, then remove them from the water with a slotted spatula. Repeat with the remaining eggs. Place 2 eggs atop each plate of watercress. Sprinkle the eggs with salt and pepper to taste, then the dill, chives and chervil. Garnish with the tomatoes.