1. Preheat oven to 425°. Heat oil in a medium nonstick skillet over medium-high heat; add breadcrumbs, pecans, and half of garlic. Cook mixture until crumbs are golden (about 5 minutes), stirring well. Remove from heat; set aside. Coat an 8- x 8-inch baking dish with cooking spray; arrange potato slices in dish, alternating between sweet and Russet. Combine half-and-half and next 5 ingredients (through thyme) in a small bowl. Stir in remaining half of garlic; whisk well. Pour half-and-half mixture over potatoes in dish. Cover dish tightly with foil; bake in middle of oven 45 minutes to 1 hour or until potatoes are tender.
2. Remove foil; discard. Sprinkle breadcrumb mixture evenly over potatoes; return to oven. Bake until crumbs are crispy, liquid is bubbling and reduced, and potatoes are tender (about 8 minutes). Let stand at least 15 minutes; serve.