Preheat the oven to 375 degrees F. Bring a pot of salted water containing the lemons, quartered onion and bouquet garni to a boil. Add the lobster to the boiling water and cook for 8 to 12 minutes. Remove the lobster from the water and place in a bowl of ice water. This will stop the cooking process of the lobster. In a sauce pan, melt the butter. Stir in the flour and cook for 2 to 3 minutes for a blond roux. Add the shallots and cook for 30 seconds. Stir in the wine and milk. Bring the liquid up to a boil and reduce to a simmer. Cook for about 3 to 4 minutes or until the sauce coats the back of a spoon. This sauce will be thicker than a normal Bechamel because it will be used as a filling. Season the sauce with salt and pepper. Remove the sauce from the stove and stir in the mustard and tarragon. Remove the lobster from the water and split the lobster in half. Remove the tail meat from the shells and with the back of a knife, gently crack the claws. Dice the tail meat and fold in the Bechamel sauce. Stir in 1/2 cup of the cheese and reseason if necessary.
Divide the mixture and spoon into the two lobster tail shells. Sprinkle the remaining cheese on top of the lobster. Place the filled lobster on a baking sheet and place in the oven. Bake for about 8 to 10 minutes or until the top is golden brown. In a hot saute pan, add the bacon and render until crispy, about 8 to 10 minutes. Add the onions and saute for 2 minutes. Add the beans and continue to saute for 2 to 3 minutes. Season with salt and pepper. Stir in the garlic. Remove the beans from the heat. To assemble, divide the bean mixture between two plates. Lay the lobster halves on top of the beans. Garnish with parsley.