In your example, I think the "roast" in roast chicken is actually a noun (correct me if I'm wrong). I guess it goes like this: when you put something in the oven, you're roasting it (verb), when it comes out it's roasted and if it is a larger piece of meat it's a roast (with the optional addition of the kind of meat it is). So there's: to roast: verb, the process of roasting roast: noun, the result roasted: adjective, a description for the state of something you roasted.

We had roast chicken = We had roasted chicken- Same meaning We are having roast chicken = We are having roasted chicken - Same We are going to have roast chicken = We are going to have roasted chicken - Same We are going to roast the chicken(cannot use roasted in this case) We just roasted a chicken and are about to eat it now.

The name of the dish can be referred to as either 'roast chicken' or 'roasted chicken'. There isn't a difference. It's more normal to use the 'roast' version, though. For example, you might say 'For Sunday lunch, we usually have roast beef with roast potatoes.' 'Roasted' is a past participle, and it refers to the process of browning and crisping in the oven. When you are giving instructions about cooking a meal, you might say, for example, 'The potatoes should be roasted for an hour a hot oven'. You couldn't use 'roast' in this case.

Roasted = usually used for food that was "processed" e.g. roasted nuts, roasted coffee. The same would work for "smoked" salmon.