Can Bok Choy be substituted for Napa Cabbage?

While Boy Chow won't be out of place in vegetable soup, it won't impart the same quality to the soup as the Napa Cabbage. I would go out and buy Napa (or even regular) cabbage. Make the Minestrone. Then with the Bok Choy, cook it as a side dish later in the week. You can slicde in 1-1/2 inch lengths and saute in a little sesame oil adding the white parts first, then the tender greens toward the end. You can sprinkle with a little soy sauce, and a little chicken stock is fine too if you want to steam it along. You are aiming for a crisp, tender finished dish. This is good in stir fries too, alongside any asian type noodle dish.

I really think it depends on your dish. If it is a stir fry then I would say go for it! I love them both and would be happy with either in most dishes.

Bok choy and nappa are two different flavors and textures, while it would be an odd addition to a minestrone it might work. You won't know until you try it.