Heat your oven to 200°C/Fan 180°C/Gas 6. Peel the squash and scoop out the seeds with a spoon. Cut into small chunks and spread out on a baking tray. Toss with 1 tbsp oil and some seasoning. Roast for 35 mins.
Peel and finely chop the garlic. Roughly chop most of the tarragon leaves. Tip both into a small bowl. Add the Dijon mustard, a little seasoning, 1 tbsp oil and 1 tbsp water. Whisk to make a dressing. Put to one side.
Heat a heavy-based frying or griddle pan till it’s smoking hot. Season the steaks all over and rub with 1 tsp oil. Fry for 1-3 mins on each side, depending on how you like your steak. Transfer to a board to rest for 5 mins.
Thickly slice the mushrooms. Add 1 tbsp oil to the steak pan. Fry the mushrooms for 3-4 mins till golden brown on both sides.
Drain and rinse the lentils. Tip into a mixing bowl and combine with half of the dressing.
Slice the steak into pieces. If there are any resting juices, pour them into the lentils.
Scatter a few spoonfuls of lentils onto each plate and top with slices of the steak, mushrooms and the squash. Drizzle generously with more dressing and garnish with any remaining tarragon leaves.