Blanch green beans in boiling salted water until just tender (about 5 minutes) then drain and shock in ice water. Repeat with yellow beans (they take about a minute less).
Toss green and yellow beans with romano beans (or if using fresh, shell the fresh beans and simmer in salted water for 20 minutes, then drain and cool). Add onion and celery (or chopped lovage).
Add vinegar, lemon thyme and olive oil and toss to coat. Season to taste and chill until ready to serve.
Salad can be dressed and chilled for up to 6 hours before serving.