Yes, you can either simmer or roast it in the roaster oven. I agree with this method from Cook's Illustrated, which is very similar to what I use, even if you start with a purchased corned beef; especially important for good veggies.

www.cooksillustrated.com/recipes/8721-home-corned-beef-with-vegetables?extcode=MASCZ00L0&ref=search_results_4

Making corned beef at home is actually quite simple. We soaked a flat-cut brisket for six days in a brine made with both table and pink curing salt and flavored with sugar, whole spices, and garlic. After the soak, the seasoning had penetrated to the core of the meat.

To break down the brisket’s abundant collagen, we gently simmered the meat in a low oven, adding carrots, potatoes, and cabbage—classic corned beef accompaniments—to the pot while the meat rested so that they simmered briefly in the seasoned cooking liquid.

(Ellen's note: this feeds 6-10)

To corn the beef yourself:
Trim all but 1/4 inch fat off flat cut 5 pound beef brisket

4 1/2 -5 pounds flat cut brisket (Ellen's note: also great for top round- takes 2 days longer because of the thickness of the piece)
3/4 cup salt (use non- iodized, such as kosher salt or sea salt)
1/2 cup brown sugar
2 teaspoons pink salt #1
4 quarts h20

3 cloves peeled garlic
4 bay leaves
5 allspice
1 T peppercorn
1 1 T coriander
Bag this all up in a large zipper plastic bag. Sits in the refrigerator 6 days, turning occasionally.

Preheat oven (or roaster) to 275 degrees for at least 20 minutes.
Rinse meat, pat dry. Place in the pan with:
3 cloves garlic, peeled
2 bay leaves
1 T peppercorn
2 quarts h20
Bring to a simmer on top of the stove.
Cover tightly, roast 2 1/2-3 hours for 5 pounds, until a fork pierces easily. Turn off the oven, or set roaster to 160.
Remove from oven/ roaster. Put meat on deep platter, 1 cup of cooking water. Return covered to off oven, or to roaster at 160, to keep warm.

In the cooking water, on top if the stove:
6 carrots (one per person)
1 1/2 to 3 pounds small potatoes,
Start on high just till it comes to a boil, turn immediately to simmer, simmer 7-10 minutes, till about 1/2 cooked.
Cut 1 head of cabbage into 6-8 wedges.
Put it in with the other veggies, turn to high just until it boils, then to simmer immediately. Then cover and simmer 12-15 minutes until the vegetables are tender.

Slice the meat, arrange on the platter with the
veggies, ladle on a little of the hot broth, and serve.

If you did the meat ahead and sliced it, you can warm it in the veggie cooking water on the way to the plates.

All tender, nothing mushy. Erin Go Bragh!