Preheat the oven to 375°F and use 1 tablespoon of butter to grease a medium baking dish.
Cook the chopped bacon in a skillet over medium heat until crisp. Remove the bacon with a slotted spoon and leave 1 tablespoon of fat in the pan.
Add the sliced leeks and minced garlic to the skillet. Cook for 4 minutes until the leeks soften.
Stir the smoked paprika, salt, and pepper into the heavy cream.
Layer half the potato slices in the baking dish. Scatter half the bacon and leek mixture over the potatoes. Repeat with the remaining potatoes, bacon, and leeks.
Pour the seasoned cream evenly over the layers. Dot the top with the remaining tablespoon of butter and sprinkle the grated Gruyere cheese over everything.
Bake for 45 to 50 minutes until the potatoes are tender and the top is golden brown. Let the gratin rest for 5 minutes, then garnish with fresh thyme sprigs before serving.