Get the veg prepped^ Preheat your oven to 180°C/Fan 160°C/Gas 4. Fill and boil your kettle. Trim the roots and any woody greens off the leeks, then halve them and rinse out any grit. Finely slice them. Rinse the mushrooms and tear them into small pieces. Snap the asparagus spears to break off the woody parts (if you bend a spear it will snap off at the right part). Chop the asparagus into bite-size chunks. Pull the tarragon leaves off any woody stalks (if the stalks are soft, you can leave the leaves on them). Roughly chop the tarragon.

Cook the veg^ Place a medium pan on a medium heat. Add 2 tbsp olive oil and the leeks. Season with a pinch of salt and pepper. Fry, stirring often, for 5-6 mins till the leeks are tender and lightly browned.

Add the mushrooms to the pan and fry, stirring often, for 6-8 mins till the mushrooms are juicy looking. While the veg fry, crumble the stock cube into a heatproof jug. Add the mustard. Pour in 200ml hot water from the kettle and stir to dissolve the stock cube.

Make the topping^ While the veg cook, tip the flour into a bowl and add a pinch of salt. Chop the cold butter and add it to the bowl. Use your fingertips to rub the flour and butter together to make a fine, sandy mixture.

Finish the filling^ Stir the asparagus chunks into the veg. Pour in the veg stock. Drain the lentils, then add them to the pan too. Stir to mix well. Add the crème fraîche and most of the tarragon (keep a pinch back for garnishing). Stir to combine and take off the heat. Taste and add a pinch more salt and pepper if you think it needs it.

Assemble the crumble^ Scoop the filling into a medium ovenproof dish. Sprinkle the crumble topping over the veg. Slide the dish back into the oven and bake for 30-35 mins till golden and bubbling. Serve straight away in warm bowls.