Testing - Roast leg of lamb with roast garlic potatoes (or garlic veg)

Roasting meat should be straightforward, and in a way, it is. The problem I've encountered are these recipes that ask for fresh herbs to be applied to the outside of the roast. The flavours in the recipe below sound right, but roasting at 400F means that the herbs end up burning on the skin. Not so nice. So how can I crust the lamb roast without burning the herbs?

The other issue was the vegetables. The potatoes cooked perfectly, but the carrots, parsnip and garlic burned. Either try to match the potato size, or add the veg later.

2kg bone-in leg of lamb
1small bunch rosemary, finely chopped
Zest from 1 lemon
3 pressed garlic cloves (save rest of bulb for potatoes)
olive oil (see instructions)
1 tsp salt
1/4 tsp pepper

1. In a small bowl combine rosemary, lemon zest, garlic, salt and pepper. Add enough olive oil just to cover and mix. Massage roast with the mixture and put directly on rack of 400F oven, with a cookie sheet underneath to catch the drippings. 1:15 hrs, until internal temperature reaches 130-140F. (WHAT part of the leg of lamb should be used to take a temperature reading???)

VEGETABLES

1.5 kg of potatoes (or mix of veg. See Instructions, below)
Garlic bulb minus 3 cloves (see above)

1. Cut potatoes length-wise, not cross-wise; they will be flat-ish and I find it looks more interesting. (MIXED VEG: this is more challenging - I think the other veg have to be added later than the potatoes. I put in parsnip and carrots, but they started to burn while the potatoes were perfectly cooked.) Put the potatoes in a large bowl. Pour over any of the remaining lamb marinade. Add more olive oil, salt and pepper, to taste, and mix to coat the potatoes.
2. When there is about 1 hour left for the lamb, spoon the potatoes in the cookie sheet under the lamb to catch the drippings. Save any olive oil mix in the bowl.
3. Husk the garlic and add to the remaining olive oil mix (and probably other veg like carrot and parsnip). Add to the potatoes 30-45 minutes before the lamb is done.