Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub and peel the parsnips and carrots. Slice them into batons about as thick as your finger. Pop them in a bowl and toss with 1 tsp olive oil. Spread out in a roasting tin. Roast for 10 mins.
While the veg roast, peel and crush the garlic cloves. Pop them in a bowl. Add the thyme leaves, 1 tsp olive oil and a pinch of salt and pepper. Stir together to form a rub.
Pop the pork tenderloin on a board and use a small, sharp knife to slash it all over. Rub the garlic and thyme mixture all over the pork.
Warm a large frying pan over a medium-high heat. Add the pork tenderloin fillet and fry for 5 mins, turning occasionally, till it's browned all over. The veg should have roasted for 10 mins by now, so take them out of the oven and add the pork to the tin. Return to the oven and roast for 30 more mins.
While the pork and veg roast, peel, quarter and core the pears. Peel and slice the red onion into thick wedges. Crumble the stock cube into a jug and stir in 300ml boiling water.
Put the pan you cooked the pork in back on the hob. Add 2 tsp oil, the pears and the onion. Season with a pinch of salt. Fry for 3-5 mins, stirring occasionally, till the fruit and veg are lightly browned and caramelised.
Stir 1 tbsp cornflour into the pan. Add 1 tbsp honey and the stock. Bring to a bubble, then turn the heat down a little and simmer for 5-8 mins till the sauce reduces a little and thickens slightly. Take off the heat and pop on a lid to keep the sauce warm.
When you're ready to serve, slice the pork and arrange on plates with the roast veg. Reheat the sauce for 1-2 mins if you need to. Spoon the pears, onions and honeyed sauce over the pork to serve.