Place the green beans in a large pan of boiling water and boil them for 3 minutes (this partially cooks the green beans, whilst retaining their vibrant colour). Drain the green beans in a colander.
Heat the oil and butter in a large frying pan (skillet) over a medium heat, until the butter starts to foam, then add the green beans, garlic, salt and pepper.
Cook the green beans, stirring often, for 3-4 minutes until just tender.
Drizzle over the lemon juice and transfer to a serving plate, then sprinkle with parmesan before serving.
Using frozen green beans?
You can use frozen green beans. Use fine whole green beans and follow the instructions in exactly the same way as for fresh green beans.
Bake in the oven instead?
If you'd prefer to bake in the oven there's no need to par-cook the beans in water first. Instead place the green beans on a baking tray and drizzle on the oil. Sprinkle with salt and pepper and cook in the oven at 200C/400F for 15 minutes. Add the garlic, toss together and cook for a further 2 minutes, then remove from the oven, drizzle on the lemon juice and sprinkle on the parmesan before serving.