Preheat oven to 350°. Bake pecans in a single layer in a shallow pan 6 to 8 minutes or until lightly toasted and fragrant, stirring once halfway through.
Meanwhile, melt 3 Tbsp. butter in a medium-size nonstick skillet over medium-high heat; add Vidalia onion, and sauté 6 to 8 minutes or until onion is tender and begins to caramelize.
Remove pecans from oven; increase oven temperature to 425°.
Combine baking mix and cheese in a large bowl; make a well in center of mixture. Whisk together milk and eggs; add to cheese mixture, stirring just until moistened. Stir in onion and pecans. Spoon into a lightly greased muffin pan, filling almost completely full.
Bake at 425° for 14 to 16 minutes or until golden. Let cool in pan on a wire rack 2 minutes. Remove from pan to wire rack, and let cool 6 to 8 minutes. Serve warm.
Vidalia-Cheddar-Apple Muffins: Omit pecans. Preheat oven to 425°. Toss 1 peeled and chopped Granny Smith apple with 2 tsp. sugar in a small bowl. Prepare recipe as directed, stirring apple into batter with onion in Step Bake: 14 min.