Preheat the oven to 400°F. Lightly coat a 1-quart casserole dish with cooking spray. In a small 8-inch skillet, melt the butter and sauté the onion until tender, then add the garlic and cook, stirring constantly, for about 30-45 seconds.

Whisk in the flour and mix well. Pour in the milk, add the pepper, and stir to combine. Continue cooking on medium heat, stirring occasionally, until the sauce begins to simmer and just starts to thicken, about 2-3 minutes. Remove from heat and set aside.

Layer half the sliced potatoes on the bottom of the casserole dish. Spread half the sauce on top of the potatoes. Use a slotted spoon to remove some of the onion from the sauce and layer it evenly across the potatoes.

Sprinkle half the cheddar, half the gruyere, and all the parmesan evenly over the first layer. Sprinkle the tarragon over the cheese layer. Top with the remaining potatoes and sauce, then sprinkle the remaining cheese evenly over the top.

Cover the casserole dish with aluminum foil and bake at 400°F for 25-30 minutes.

Uncover and continue baking for another 15-20 minutes, or until the potatoes are fully cooked through.