Preheat the oven to 180°C/350°F/gas 4. Sit the chicken and garlic bulb in a large sturdy roasting tray, sprinkle with a pinch of sea salt and black pepper, drizzle with 1 tablespoon each of red wine vinegar and olive oil and rub all over the bird. Roast for 1 hour.
Take the tray out of the oven and use tongs to move the chicken to a plate, laying the bacon over the breasts. Use a potato masher to squash and squeeze the soft garlic out of the skins in the tray, then toss the mushrooms in the juices, tearing any larger ones. Return the tray to the oven. Sit the chicken directly on the bars above and roast for another 20 minutes, or until everything is golden and cooked through.
Use a slotted spoon to transfer the garlicky mushrooms to a serving platter, sitting the chicken on top to rest. Place the tray of juices on a high heat on the hob, add the spinach, then drain and add the lentils. Stir until the spinach has wilted, then, off the heat, ripple through the crème fraîche and season to perfection. Pick the tarragon leaves over the chicken, and serve it all together.