Now that Trader Joe’s has all-butter puff pastry back in stock, it’s a snap to make this tart, 1/4 of which is pictured above. That’s how fast we ate it before I realized I should take a photo.
Roll out the defrosted pastry (8 or 9 ounces) on a silpat or parchment or directly on a baking sheet to almost double the original size. (I like my pastry really thin. If you want it thicker, roll it out more moderately.) Fold over the edges to raise them and pinch or seal with a fork. Prick the middle well with a fork. Preheat oven to 375 degrees.
Thinly slice and coarsely chop one yellow onion and caramelize with a bit of fat. (15 minutes). Peel and thinly slice (on a mandoline) 2 large yellow/gold potatoes. Toss the potato slices with 1 scant Tbs. minced fresh sage, 1 tsp each salt and pepper, 2 Tbs. sour cream and 1 ounce of shredded gruyere. Stir in the caramelized onions and heap this mixture onto the prepared pastry. Bake on the lowest rack of the oven for about 45 minutes. (Liquid might drip out so put a rimmed pan under it?) The potato slices will cook into less than half volume as they bake, without becoming mushy. Lift the bottom of the tart with a spatula to see if it is baked enough–the bottom should be golden brown and crisp. You could add cooked bacon or substitute other herbs.