Wash lemon and use a microplane to remove the zest. Roll the zested lemon on the counter to help release the juice. Slice it in two and use a reamer or citrus juicer on one half of it to get the necessary 1-2 tablespoons.
In a medium bowl, combine the mayonnaise, lemon zest, lemon juice (start with 1 T), dijon mustard, 1/4 tsp Kosher salt and a dash of sweet paprika. Stir with a whisk to combine.
On a cutting board, chop lobster meat into large bite size pieces. Finely chop the celery. Add both to the large bowl with the sauce. Using a spatula, toss gently to combine all the ingredients, making sure to get all of the sauce off the bottom. Taste and season to taste with additional lemon juice, mustard salt and/or paprika. Gently stir in green onions. Cover and refrigerate until ready to use.
Split buns (if needed) and spread both cut sides with softened butter. Heat a large nonstick skillet over medium high heat. Toast buttered sides in the pan until desired degree of golden brown and crispy. Set aside on a platter. Repeat with remaining buns.
Divide the lobster salad between the buns. Garnish with a sprinkle of green onions and paprika if desired. Serve with coleslaw.