Put the stock or white wine into a small saucepan with the bouquet garni, bring to the boil then simmer until the liquid has reduced by half.
Top and tail the green beans, cut into 2.5-cm lengths and put aside. Drain and rinse the canned beans and put aside.
Cut the pancetta or bacon into small cubes. Fry in a large frying pan for 1–2 minutes until crisp, then drain on paper towels.
Peel and chop the onion and fry in the remaining fat in the pan over medium heat until golden.
Add both lots of beans and the reduced stock or wine to the frying pan. Bring to the boil, cover, then reduce the heat and simmer for about 10 minutes, until the green beans are tender. To test, cool a bean under cold running water, then bite into it. When the beans are ready, there should be almost no liquid left in the pan.
Meanwhile, chop the tarragon or parsley. Add the crème fraîche to the beans, warm through, then add the pancetta or bacon, the herb, and salt and pepper to taste. Warm through again, then serve.