The carrots and potatoes need a longer roasting than the other vegetables. Also, roasting onion keeps it with a relatively "raw" flavor; if you want your onion to taste cooked and lose its sharpness, you should sweat it in oil on a stove before roasting in the case of eating the stuff roasted. If you are going to cook this stuff to a soup, don't roast the onion at all, just sweat it in the pot and then add the roasted vegetables to cook the soup.
Generally, roasting time for nonstarchy vegetables isn't that long. I would start with just the carrots and potatoes (carrots are not starchy, but they are woody and need a long time for the cell walls to break apart and soften; potatoes need some time roasting after the internal temperature has reached 70°C because of the starch). The garlic can also use a long roast, but I would cut it very fine and sweat it together with the onion. That depends on whether you prefer garlic-flavored oil infusing your soup or roasted cloves to bite on.
After the potatoes and carrots have been in the oven (tossed with a little oil) for 25-30 minutes (preferably covered, so they half cook, half roast in their own steam), I would add the tender vegetables (if you want to put the sweated onion in, that's the time for it too). Remove the cover to let the vegetables roast nicely. Then leave in the oven for another 25-30 minutes or until tender.
You won't get the strong roasted flavors of, say, a pepper roasted directly on a flame, but you will have some nicely cooked veggies. You can then cook them up for a very short time in a broth to make a soup (under a minute), or use them in other ways, for example as a side dish, or extending the recipe to become a casserole.
The two-step addition of the vegetables is important, because the tender vegetables like sweet peppers and zucchini become mushy when overcooked and dry when overroasted.
If you want to add spices, add them in the beginning, mixed with the oil. Dried herbs can also be added at the beginning, to have time to rehydrate a bit. Don't add fresh herbs at the beginning, they will wilt. Add them 3-4 min before the final dish (i.e. the soup) is ready.
With all due respect to Chef Oliver, I do see some problems with the recipe he offers.
In The New Best Recipe Book, from Cook's illustrated in the section on beef roasts they note that when beef is roasted at above 250°F the internal temperature will rise by 18°F during the resting period, however when roasting below 250°F the internal temperature only increases by 3°-5°F. For this purpose lamb is comparable to beef and the result you experienced was to be expected. IMHO the oven temp of 400° is too high, and I would recommend going lower and slower. Try 225°F, and plan on it taking much longer to cook. Continue to roast in the oven until an internal temp of 140°F is reached. This technique will afford you a much more tender and juicy roast, even if it takes longer to get there.
You are correct in directing some attention to the stuffing as well. Where the recipe asks you to add lemon juice if it the stuffing is too dry, I would suggest adding lemon juice until the stuffing is 'very moist' (not sure how to quantify that for you).
Tying the twine too tight is unlikely to have had an impact, but your choice of basting wine may have. Go with something fruitier/sweeter in flavor.
Best Answer
Slow roasted vegetables are delicicous. But the accent is upon slow - at 100 degrees you could be talking 6-8 hours. It's very similar ti slow cooked lamb, use lots of olive oil and wait.
I like to put in slices of potatoes for the first three hours and then add pepper courgettes, and other vegetables needing less cooking time for the kast 3 hours.
Great to serve with a half peanut butter, half feta cheese thick dip and some crispy toasted pitta bread.