Preheat the oven to 400°F.
Spread the bread pieces on a rimmed baking sheet and bake for 8-10 minutes. Stir the bread and spread it back out again. Baking another 8-10 minutes, or until the bread is completely toasted. Set aside when done.
While the bread is toasting, heat the oil in a large (5-quart) pot over medium-high heat until shimmery. Add the onions and celery and saute until just softened, about 4-5 minutes. Season lightly with salt and pepper while they cook. Add the garlic at the end and stir for 30-60 seconds.
Turn the heat down to medium. Add the butter to the pot and let it melt (don't let it brown). Then, stir in the onion flakes or powder, garlic powder, celery seed, crushed parsley, sage, thyme, 1 teaspoon salt, 1/2 teaspoon pepper, and chicken broth. Turn the heat up and bring to a boil, then remove from the heat.
Gently stir in the toasted bread pieces, until coated. Cover with a lid and let sit for 5-7 minutes, until most of the liquid is absorbed.
Use a fork to gently fluff the stuffing before serving. Serve "as is", or transfer to a 9x13 casserole dish and bake at 350°F for 20 minutes, if you want it to be crispy on top.