I just saw this on RecipeZaar. I haven't tried it, but it looks good.

Baked Fish Plaka-Style (Greek) Recipe #391864
A great crockpot recipe for fish; you've got to really like onions, though (I do)! Came from a book of ethnic recipes for the slow cooker.
by L'ecole
2 hours | 10 min prep

SERVES 4 -5

4 medium onions, thinly sliced
3 tablespoons olive oil
2 tomatoes
1/2 cup fresh parsley, finely chopped
2 garlic cloves, minced
1 1/3 lbs white fish fillets (turbot, sole, etc.)
2 tablespoons lemon juice
salt and pepper
1 lemon, thinly sliced
In a large skillet, saute onions in oil until golden brown and almost caramelized.
Peel and chop 1 tomato. Add to onions, along with parsley and garlic. Simmer for a few minutes longer.
Place fish fillets in a lightly buttered crockpot. Sprinkle with lemon juice, salt and pepper.
Spoon sauteed vegetables over fish.
Slice remaining tomato and place on vegetables.
Top with lemon slices.
Cover and cook on low for about 1-1/2 hours, or until fish flakes when tested with a fork.
Nice over rice!