For the pickled mustard seeds: In a small pot over high heat, bring the white vinegar to a boil. Add the mustard seeds, then immediately remove from the heat and let rest for about 30 minutes. Strain seeds, reserving vinegar for another use. Set aside.
For the lobster: Bring a very large pot of generously salted water to a boil. Place the lobsters head first into the water and cook at a rapid boil until bright red, 5 to 6 minutes. Remove the lobsters from the water and set them aside to cool.
For the leek stuffing: Slice the leeks into 1-inch pieces and place in a large mixing bowl. Fill the bowl with cold water and, with your hands, agitate the water to dislodge any dirt and grit from the leeks. Let the grit settle to the bottom of the bowl, then use a spider or slotted spoon to gently lift the leeks out of the water and place in a colander set in the sink to drain.
Heat the olive oil in a large saute pan over medium heat. Add the butter, leeks, a pinch of salt and a few grinds of pepper. Cook, stirring often, until the leeks are tender, 6 to 8 minutes. Stir in the mustard and red wine vinegar. Remove from the heat and set aside.
For the assembly: Preheat the oven to 400 degrees F.
Once the lobsters have cooled, cut them in half and take the tail and claw meat out. Cut the meat into bite-size pieces. Set aside.
Clean out the entire inside of the lobster shells, discarding the innards and the claw shells. Divide the leeks among the empty lobster shells, then top with the lobster meat. Brush the lobster with the melted butter and finish with some pickled mustard seeds. Bake until heated through, approximately 10 minutes.