Blanch the fava beans in boiling water for 3 minutes. Drain well. When the beans are cool enough to handle, remove their skins. Set the beans aside.
Heat the canola oil in a large skillet over low heat. Add the onion and cook until transparent but not brown, about 5 minutes.
Raise the heat to medium and add the cucumber. Season lightly with salt and pepper. Cook until the cucumber is tender, about 6 minutes. If the onion begins to brown, add a tablespoon or two of water and continue cooking.
Add the corn and cook 1 to 2 minutes. Add the fava beans and bell pepper. Season and cook 2 minutes more, until tender. Stir in the tarragon and serve.