Tricolor Vegetable Terrine

1 small (5-ounce) sweet potato

2 small (about 1/4 pound) red beets , without tops

1 tablespoon(s) butter

1 medium onion , chopped

1 package(s) (10-ounce) frozen green peas

1 clove(s) garlic , chopped

2 teaspoon(s) fresh (or 1/2 teaspoon dried) thyme leaves

1/2 teaspoon(s) salt

1/4 teaspoon(s) ground black pepper

1 cup(s) fresh parsley leaves

3 large eggs Heavy cardboard Fresh thyme sprigs