Cut the cauliflower into 1.5 cm thick steaks (reserve any trimmings for another use).
Brush the steaks with olive oil and season lightly with salt.
Heat a griddle pan over medium-high heat and cook the cauliflower on both sides until lightly charred and golden. Set aside.
Meanwhile, heat a little olive oil in a large pan and sauté the onion for 5 minutes, until soft.
Add the garlic and cook for 1 minute until fragrant.
Stir in the lentils, both mustards and the vinegar or lemon juice. Cook for a minute, then pour in the cream. Season to taste.
Nestle the cauliflower steaks into the sauce, cover with a lid and cook gently until the cauliflower is tender and fully cooked. Add a splash of water if the sauce becomes too thick.
Scatter on top the chopped parsley just before serving.