Peel the potatoes and slice them into 1/8-inch rounds, preferably on a mandoline. Heat grapeseed oil in a large saute pan and cook onion it begins to turn translucent, and stir in garlic. Then add sliced potatoes and stir to coat with oil and onion mixture, and allow to cook for a few minutes. Reduce heat and stir in heavy cream. Cover and let cook slowly over low heat until potatoes are tender. Season generously with salt and pepper and stir in fresh chopped tarragon. Remove from heat and set aside to rest and cool.
Preheat oven to 375 degrees F. Dust work surface with flour and roll out 2 sheets of puff pastry and cut to the diameter of a metallic pie plate. Shape 1 sheet of the pastry to the dish and add the potatoes. Fill the pastry with the cooled potato mixture. Place the other sheet of puff pastry on top of the pie dish, and seal and flute edges. Brush the top with egg wash.
Put the dish on a baking sheet on the oven's center rack and bake until crust is golden brown, about 25 minutes. Allow pie to set up for about 10 minutes before slicing.
While the pie is setting up, steam the haricots verts. Heat the clarified butter in a skillet and saute the garlic. Briefly saute the haricots verts with the garlic, season with salt and pepper and remove from heat.
Cut pie into wedges or squares, transfer to serving plate, and arrange some haricots verts in a small bouquet at the pointed end of the wedge or square.