In a large resealable plastic bag, place the garlic and the lime juice.
Add the roast; turn to coat with the marinade.
Refrigerate overnight, turning the bag occasionally.
When ready to slow cook, place the roast in a three and a half-quart or larger slow cooker; discard the marinade.
Add the broth, seasoning and tomatoes.
Cover the pot, and cook on low for eight hours.
Remove the roast to a serving platter; cover to keep warm.
Turn slow cooker to high.
In a small mixing bowl, combine the cornstarch and water until smooth.
Add cornstarch mixture to sauce, stirring well to mix.
Cook an additional 15 minutes until the sauce thickens.
Pour the sauce over the roast and serve.