1 pound Romano beans, cut on the diagonal into 1 1/2-inch lengths
1 pound yellow wax beans, cut on the diagonal into 2- to 3-inch lengths
1/2 pound radishes, sliced into thin rounds
In a medium bowl, whisk together the vinegar, mustard, mustard seeds and shallots. Whisk in the olive oil in a slow steady stream. Add the thyme and season with salt and pepper.
In a large pot of boiling salted water, cook all of the beans together until just tender, about 8 minutes. Drain the beans and plunge them into a bowl of ice water to stop the cooking. Drain again and pat thoroughly dry with kitchen towels.
In a bowl, toss the beans and radishes with the dressing. Season with salt and pepper and serve.
The dressing and beans can be refrigerated separately for up to 1 day.