Using nonstick cooking spray or butter, grease the sides and bottom of a 9x13-inch baking dish. If using individual size baking dishes, grease them accordingly.
Peel potatoes and slice thinly.
Remove seeds from jalapeños and dice finely.
Cut off fibrous end of leek. Make two cuts lengthwise across leek, keeping the root intact. Thoroughly wash the leek layers under running water to remove sand and dirt. Thinly slice the light green and white portion of the leek, discarding the root.
Place butter in large saute pan or cast iron skillet set on medium high heat. Add jalapeños, leeks, salt, and pepper, and sauté for 4-5 minutes. Add the cream and half & half and bring to a simmer.
When cream mixture is hot, pull from stove, add cheeses and blend in thoroughly.
Alternately layer the potato slices and cheese sauce into the prepared casserole dish until the dish is full, topping with the remaining cheese sauce. If using individual dishes, do the same in each of them.
Cover dish(es) with aluminum foil and bake at 350°F for 1 hour (this may be less time for the individual dishes). Remove foil and bake another 15 minutes, or until the top is golden brown.