Preheat the oven to 375 degrees. Line 3 sheet pans with parchment paper.

In an electric mixer fitted with the paddle attachment, beat the butter, brown sugar, and granulated sugar on medium-high speed for 3 minutes, until light and fluffy. Scrape down the bowl with a rubber spatula. On low speed, add the vanilla, then the eggs, one at a time. Scrape down the bowl again.

Meanwhile, sift the flour, baking soda, and salt into a medium bowl. Mix in the oats. With the mixer on low, slowly add the flour mixture to the butter-sugar mixture. Dont overbeat it! With a rubber spatula, stir in the chocolate and cranberries until the dough is well mixed. With a 1 3/4-inch ice cream scoop (or two spoons), scoop round balls of dough onto the prepared sheet pans. Sprinkle lightly with fleur de sel. Bake for 10 to 12 minutes, until nicely browned. Serve warm or at room temperature.

Note: If you prefer cookies thin and crisp, bake them straight from the mixing bowl. If you prefer them chewy in the middle and crisp outside, chill the balls of dough.

MAKE IT AHEAD: Scoop balls of dough, place in sealed containers, and refrigerate for up to a week or freeze for up to 3 months. Defrost and bake before serving. Baked cookies can be stored in plastic bags and reheated for 5 minutes at 350 degrees.