The flavor is in the oil - the garlicky, smoky goodness is transferred to the oil by cooking it in the oil slowly, and this is what permeates the whole dish.
You'll want to cook this dish a LOOONG TIME and very SLOWLY. Roasted garlic may do the same thing, but it won;t permeate the oil as will garlic cooked slowly in the oil will do. Also i never encountered roasted garlic in traditional italian cooking - in fact, i;ve never encountered it here in italy at all. You;re talking about a traditional italian dish, so i would suggest using a traditional italian technique. I can give you some suggestions from how i do cauliflower pasta, which has the same effect, nice, roasted garlicky taste permeating every bite. then transport it to your dish:
I cut the cauliflower in small flowerettes, and the bigger ones i cut lengthwise up the stem in half or quarters.
Put the oil in a heavy frying pan - don't be just filming the pan, you'll need a little depth - think of making pasta aglio olio - the oil carries the flavor to the pasta, so that in between cauliflower pieces there is flavor. So put enough oil that if you tilt the pan there is a nice puddle in the corner, (but not so much that it looks like a deep puddle when the pan is flat!)
For about four people Add five or six cloves of garlic (mine here are very big, you may need more) sliced (not mashed or chopped, just slices, maybe the thickness of a penny - they;re going to cook a long time)
Add red pepper flakes or a few tiny whole hot peppers.
Add the cauliflower (yes, all together with the garlic, but in that order, the garlic and hot pepper is sitting in the oil, cauliflower on top). Add salt to taste.
Turn on heat to LOOOOWWW. Cover. and let it cook very very slowly, mixing occasionally, until the cauliflower gets soft, really really soft. It and the garlic will become barely nut brown in places, the color of caffelatte. Cauliflower will be mushy. Garlic will be soft and barely nut brown. There;s your sort of nutty smoky taste. If you blanch the cauliflower first, you will cut down too much on the time it sits in the oil and develops its flavor.
This is the method usually used here for pasta dishes. Now broccoli is slightly different and cooks differently, and has to be peeled, each stalk, each stem, and the tops can be problematic because they';re usually not as tender as cauliflower. Broccoli rabe is even more so, needs to be peeled and sliced up the length of the stalks and they cut into pieces no longer than the pasta you use. Generally people blanch it first, but it depends on how tender it is. I would prefer, if using it to flavor pasta, to cook it the way i do cauliflower and after the first five minutes, add a little water and cover, cooking slowly, so that all the flavor goes into the oil, not drained into the sink. If it's blanched first, you won;t get the same effect.
By the way, try the cauliflower this way. I keep telling people and i think they think it;s not appealing. I've made this for people who hate cauliflower and they loved it. You mix it directly with the freshly drained pasta, make sure it's well mixed, and then mix in parmigiano.
It;s the slow slow cooking that does the trick.
let me know if you try it.