Reduce the white wine by cooking it over high heat in a large nonaluminum pan until there is half the amount you started with. (Lower the heat, if necessary, to keep it from boiling over.) Pour the reduced wine into a mixing bowl and add the olive oil. Put the lemon juice in the pot you used to reduce the wine. Whisk in the cornstarch, warm it over medium heat, and cook, stirring, until thickened. Add the wine and olive oil mixture to the thickened lemon juice and reheat, stirring constantly, until the mixture boils and thickens but is still the consistency of cream soup. Remove from the heat. Add the garlic and capers and season with salt and pepper. The dressing should be quite tart. Adjust any of the ingredients according to your taste. Let the dressing sit for 2 hours, if possible, to give the flavors a chance to blend.
Cook the green beans in boiling water for 5 to 8 minutes with the lid off the pot, until tender but not mushy. Drain and dump into a bowl filled with ice water. When chilled, drain immediately and place into a bowl lined with a towel to dry even more. About 10 minutes before serving, toss the beans with the dressing.