1. Preheat oven to 500°F. Place pizza stone on bottom rack of oven; heat for at least 30 minutes.
2. Crust: Meanwhile, pour water into bowl of stand mixer. Add sugar; stir until sugar is dissolved. Sprinkle yeast over top; let stand for 5 to 10 minutes or until foamy. Add 2 cups of the flour and salt; mix using dough hook attachment. Gradually knead in enough of the remaining flour until ball forms. Transfer dough to floured countertop; knead for about 8 minutes, adding flour as needed to make smooth dough.
3. Place dough in greased bowl; cover with tea towel. Let rise in warm spot for about 1 hour or until doubled in bulk. Knead dough briefly; form into ball. Cover with clean tea towel; let rest for 30 minutes.
4. Toppings: Slice or tear mushrooms. Heat oil in large skillet set over medium-high heat; cook mushrooms, shallots, garlic, thyme leaves, salt and pepper, stirring frequently, for 8 to 10 minutes until mushrooms are browned and tender. Deglaze pan with sherry; cook for about 1 minute or until liquid has evaporated.
5. Form dough into 14-inch round. Spread cornmeal on pizza peel; arrange dough on top.
6. Sprinkle mozzarella and 1 cup Gruyère cheese over dough. Top with mushroom mixture and remaining Gruyère cheese.
7. Slide pizza onto pizza stone; bake for 13 to 15 minutes or until cooked as desired.
8. Top with drizzle of truffle oil and sprinkle of chopped chives.
Use store-bought pizza dough if desired.
The dough can be prepared the day before and proofed overnight in the fridge.