Cook frozen string beans in a little salted water just until tender crisp. Drain and set aside. Keep covered.

In a large frying pan, heat one tablespoon olive oil for the crunch. Add the pecans and walnuts; cook over medium heat until toasted, about four minutes.

Stir in maple syrup and salt. Cook and stir for three to three and a half minutes or until pecans are glossy and fragrant. Turn onto parchment paper to cool.

Wipe out the frying pan and heat the lemon-infused olive oil. Add onion and sauté until tender. Add the flour and stir until onion is coated. Add the remaining ingredients and stir with a whisk until thickened and smooth. Remove from heat.

Mix string beans into sauce and stir until lightly coated. Transfer to a serving bowl and sprinkle with nuts.