Prepare a medium-hot fire in a charcoal grill with a cover, or preheat a gas grill to medium-high.
Make the patties: mash the peas in a large bowl using a fork.
Add in the onion, egg, salt, pepper, ketchup, and Worcestershire sauce; mix well.
Divide the mixture into 6 portions and form the portions into patties to fit the buns.
Lightly brush the patties on both sides with oil, then lightly dust with flour.
When the grill is ready, brush the grill rack with vegetable oil; place the patties on the rack, cover and cook, turning once very carefully, until well browned, firm to the touch, and no longer gooey when cut into with a small, sharp knife, 5-6 minutes on each side.
During the last few minutes of cooking, place the buns, cut side down, on the outer edges of the rack to toast lightly.
To assemble: spread the cut sides with mustard, mayonnaise, and ketchup.
On each bottom bun, place a patty, 3 pickle slices, a tomato slice, and a lettuce leaf; add the bun tops and serve.