In a 10 inch nonstick skillet, heat 1 tablespoon olive oil over medium high heat. Add the onions, then cook 2-3 minutes, until opaque and fragrant. Stir in the pancetta, cook 5-8 minutes, or until pancetta is crispy and onions are caramelized. Remove from heat and stir in spinach. Using a paper towel, dab remaining olive oil over bottom and sides of a 13 X 9 pan. Arrange bread cubes in pan to completely cover bottom. Spoon onion pancetta and spinach mixture, with drippings from pan, over bread. In a large bowl, whisk together mascarpone, cream and eggs, adding one egg at a time, and whisking briefly between each addition. Do not over mix. Season with salt and pepper to taste. Pour egg mixture over bread. Spoon peach preserves by evenly spaced tablespoonfuls over top of the eggs. Sprinkle with basil leaves. Cover tightly with plastic wrap, then refrigerate 2 hours or overnight, but no longer. Heat oven to 350 degrees. Remove strata from refrigerator and uncover. Bake 30-45 minutes, or until a knife inserted in center comes out clean. Serve warm.