1. Heat oven to 350 degrees F. Coat a 2-quart oval baking dish with nonstick cooking spray. Combine dried porcini mushrooms with boiling water; soak 20 minutes, then drain and chop.
2. Blend finely grated Parmesan, mascarpone, half-and-half and a pinch of nutmeg; set aside.
3. Heat butter and canola oil in a large nonstick skillet over medium-high heat. Cook garlic cloves 30 seconds, then add mushrooms. Cook 3 minutes.
4. Carefully slice potatoes into 1/8-inch-thick slices using a mandoline or V-slicer.
5. Spread 1/4 of the potato slices over bottom of prepared dish, overlapping as necessary, then sprinkle with salt. Sprinkle with 1/4 of the mushrooms. Repeat layer, then top with half of the cheese mixture. Repeat with remaining potatoes and mushrooms, forming 2 layers, then finish with remaining cheese mixture. Sprinkle with Gruyere cheese.
6. Bake at 350 degrees F for 1 hour or until top is browned and bubbly.