Heat the olive oil on medium heat in a large, thick-bottomed Dutch oven (5 to 6 quart).
Add the onion and the bay leaves and cook for 2 to 3 minutes, until slightly softened.
Add the chopped celery and cook for 2 to 3 minutes more.
Add the minced garlic and cook for a minute more.
Add the chopped pumpkin and the ham hock to the pot.
Stir in the chicken stock, tomatoes, and thyme.
Increase the heat to bring to a simmer.
Reduce heat to maintain a simmer, uncovered. Let simmer for an hour and a half.
Remove the ham hock from the soup pot to a plate to let cool enough to handle.
Add the white beans and cook for 15 minutes.
Add the chopped chard and simmer until the chard is wilted, a few minutes more.
Strip the meat from the ham hock, chop it, and return to the pot.
Add 1 to 2 cups of water to the soup to thin it to your preference.
Remove the bay leaves before serving.
Add freshly ground black pepper and adjust seasonings to taste.