Vegetarian shepherd's pie

4 large floury potatoes, peeled and cut in quarters

55g butter, plus extra for greasing

100ml milk

20ml olive oil

1 garlic clove, minced

1 small onion, chopped

300g sliced button mushrooms

2 large carrots, peeled and sliced into disks

1 head of cauliflower, cut into small florets

1 courgette, sliced and cut into disks

250g frozen peas

1 tin of diced tomatoes

125ml vegetable stock

2 tbsp dried thyme

2 tbsp dried rubbed sage

Hot red pepper chilli flakes, to taste

Sea salt and fresh ground pepper, to taste

125g parmesan cheese