Boil the green beans for 3 minutes until just tender (or to your taste), then drain and run under cold tap water (to stop cooking further). You could also steam.

Transfer to clean tea towel and pat dry. Set aside.

Line tray with 3 - 4 layers paper towels.

Toss onions with flour, panko, salt and pepper.

Heat about 2cm / 3/4" oil in a heavy based skillet over medium high heat. Dip the end of one onion in - it should sizzle satisfyingly immediately.

Add some onions, not too many (see video), then fry for 1 1/2 minutes or until LIGHT golden brown.

Transfer to tray, spreading out (don't pile up). (Note 3)

Repeat with remaining onion (5-6 batches). Set aside to cool, DO NOT cover.

Melt butter in a skillet over medium high heat. Add mushrooms and cook for 1 minute.

Add salt and pepper, cook until mushroom starts to soften (1.5 minutes) then add garlic and cook for 1 minute until garlic is golden.

Turn heat down to medium, add flour and mix until you can't see flour anymore.

While stirring constantly, gradually pour in half the broth. When flour lumps are dissolved, pour in remaining broth and cream, stir.

Add nutmeg, stir.

Cook for 5 minutes, stirring casually at first then regularly as it thickens, until sauce thickly coats back of wooden spoon and you can draw a path across it (see video). Sauce will not thicken further in oven.

Preheat oven to 350°F / 180°C (160°C fan).

Pour beans into pot they were boiled in (or use bowl). Pour over Mushroom Sauce, stir.

Pour into 2.5 quart / 2.5 ltr casserole dish (Note 4), smooth surface.

Sprinkle with onions (might not use all!), then bake for 15 minutes until bubbling on the edges and the onions are deep golden. CHECK at 10 minutes to ensure onions are not burning (Note 5).

Serve immediately! (See notes for make ahead)