Heat oil in a large oven-safe casserole, brown ducks in batches. Season upon removal with salt and pepper, transferring to a bowl or platter, cover loosely with foil, set aside.
Remove all but one tablespoon fat, discard or reserve remainder, for another purpose.
Cook onions till golden, stirring often. Add garlic and thyme, cooking 1-2 minutes more.
Boil in port and wine till reduced to a glaze.
Add broth, tomatoes and lemon peel, taste and correct for salt and pepper.
Return duck to casserole, cover, and place in the middle of a heated 350 degree oven. Bake until the duck is quite tender, 80-90 minutes. Remove, put duck on a platter and cover to keep warm. Boil juices till reduced to a quart, about 20 minutes. Add chard, cover, simmer gently till tender. Return duck to pan to warm through.
Serve immediately, if desired, with potatoes, sautéed in some reserved duck fat.