Borscht - or what to do with leftover beets and potatoes

The surfeit of leftover vegetables from this past weekend's Harvest Jazz & Blues Festival meals was weighing on my mind (and occupying way too much fridge real estate). Yesterday was a crisp fall day and memories of delicious borscht sprang to mind (red fall leaves - red beets, you see the link?). A perfect solution to my veggie l/o dilemma.

(Almost) Vegetarian Borscht

2 tbs butter
1 large onion, chopped
900 mL container of beef broth
1796 mL (28 oz) can of diced tomatoes, undrained
2 Tbs tomato paste
4 c coarsely chopped or shredded cabbage
3 large cooked carrots chopped
3 cups small red potatoes, cooked and halved
4 cups chopped beets, cooked
2 tbs minced garlic
2 tsp salt
1 tsp fresh ground pepper
2 tbs sugar
1/4 cup white vinegar
2 tsp caraway seed
1 tsp dill seed
1 cup water
2 sachets low-sodium beef oxo
3 halves sun-dried tomatoes, coarsely chopped
chopped mushrooms (optional)

In very large pot, melt butter and saute onions until transparent. Add remaining ingredients in order and bring to low boil, stirring occasionally. Simmer over medium heat for about 1 1/2 hours, again stirring occasionally. Serve with dollop of sour cream.