Season the brisket with 3/4 teaspoon salt and the pepper. Put it in a large slow cooker with tomatoes and garlic.
Cover and cook on low for about 10 hours, until very tender.
Near the end of cooking, heat the oven to 425°F. Toss both sliced onions with olive oil and the remaining salt. Spread on a tray and roast for about 20 minutes until browned.
Slice or shred the tender brisket, spoon over its sauce, and finish with the roasted onions and parsley.
Inspired by The Good Housekeeping Test Kitchen.Estimated nutrition is for one of 6 equal portions. The estimate uses trimmed raw brisket. The amount of visible fat removed can change the result.Storage: Refrigerate perishable portions in shallow, covered containers within 2 hours, or within 1 hour above 90°F. Keep the refrigerator at 40°F or below and use cooked leftovers within 3–4 days. If reheating cooked leftovers, heat them to 165°F throughout.