This is one of those messy dishes best eaten in a casual setting where finger licking is acceptable and there are plenty of napkins on hand.
Preheat oven to 350F. Oil bottom of a large casserole with a cover.
Remove stems and all outer leaves from artichokes. Pull out cone of tender inner leaves, and use a small spoon to scrape away the thistly choke from the center. Trim away any dark green bits from bottoms. Put trimmed artichokes into acidulated water.
Melt butter in a saucepan over medium-high heat. Add onions, and cook, stirring often, until softened, for 3 to 5 minutes. Add garlic. Cook for 1 minute more. Transfer to a bowl, and add breadcrumbs, cheese, oregano and parsley. Use a fork to blend into a well-mixed loose stuffing, and season with salt and pepper.
Drain artichokes, and place on a work surface upside down. Press down gently with heel of your hand to loosen leaves. Using fingers, pull leaves apart and fill insides with small amounts of stuffing, distributing stuffing evenly among artichokes.
Arrange stuffed artichokes snugly in casserole. Sprinkle chopped tomato over top, and cover.
Bake until a skewer easily pierces through artichoke from top to bottom, for about 45 minutes. Remove from oven, cool slightly and serve in individual bowls.
A light- to medium-bodied Italian red such as Castello di Gabbiano Chianti or the Tosco Forte Sangiovese di Toscana would be a nice partner to this dish.