Heat a griddle pan over a high heat until smoking. Slice the bread into 2cm slices and place on the griddle pan, turning after 1 minute or until each side has been gently chargrilled. You will need to do this in batches. Set aside.
Plunge the beans into a pan of boiling, salted water and simmer for 2 minutes, then drain and run under a cold tap until cool.
Pop the beans out of their skins, then season. Pick, roughly chop and stir through the tarragon leaves. Gently crush with a fork, so you have a mix of whole and crushed beans. Drizzle with the oil.
Halve the garlic clove and rub onto each slice of the griddled bread.
In a separate bowl, mash the feta with the yoghurt to form a paste, season and thickly spread onto each garlicky slice, then top with the beans. Drizzle with oil from the bowl.