Melt 2 Tbsp. butter in a large skillet. Add the onions and carrots sprinkle with salt and pepper. Cook the onions over low heat, until caramelized and tender, 20-25 minutes. Stir in the celery and cook for an additional 2-3 minutes until the celery is translucent but still fairly crunchy. Remove the skillet from the heat and set aside.
Meanwhile, toast bread slices until lightly browned in a toaster. While the bread is still hot, lightly butter both sides of each piece with remaining 2 Tbsp. butter. Cut into 1-inch squares and transfer them to a large bowl. Toss with oregano, sage and pecans. Mix to blend, stir in the onion mixture. Add chicken stock to moisten all of the ingredients.
Butter a 6-cup casserole dish with any remaining butter. Transfer the stuffing to the prepared dish. Cover with aluminum foil and bake 20 minutes. Remove foil and bake an additional 10 minutes or until top is lightly browned and stuffing is heated through. Serve immediately.