Preheat the oven to 200C (fan).
In a large bowl mix together the mustard, garlic oil, honey, lemon zest, lemon juice, salt and pepper.
Add in the chicken thighs - mix them into the marinade. Allow to sit whilst you prepare everything else. This could be for 15 minutes or could be as long as you want, even overnight.
Hasselback your potatoes - place your potatoes onto a wooden spoon and use a sharp knife to cut slits, the spoon means you won’t go all the way through.
Peel and chop the carrots and parsnips into 1cm thick chunks; place them into a bowl with the potatoes and drizzle with some garlic oil. Mix them to coat.
In a large roasting tin, place the chicken and dot your veggies in the gaps. Place into the oven for about 35-40 minutes until the chicken is starting to go golden and the veg is almost cooked.
Remove from the oven. If there's excessive liquid in the bottom of your pan, carefully drain it before continuing. Next, sprinkle on the frozen peas and drizzle with any leftover marinade left in the bowl. Cover with foil and cook for a further 10 minutes.
Check everything is cooked and serve it up straight away or with gluten-free gravy.