I use whole salmons with the heads cut off and the insides cleaned out. You could use salmons with the heads still attached if you prefer, but the guts gotta go!
I spice the fish inside and out before I insert the asparagus. If you wish to use a little lemon for flavoring spread the lemon first then the spices and herbs.
I used: Tarragon Basil Garlic (granulated) Onion (granulated) Ginger (granulated or powdered) Paprika You can also add a little of any or all of: Cumin Sage Curry Savory Celery seed (preferably ground) Dill weed (not seed)
Cook the asparagus first if you wish. If you don't precook it at all it will come out almost raw. How long you precook it depends on how firm or soft you want the asparagus. I prefer it slightly cooked and looking dark green. It will cook less in the fish than you might think so undercook it in advance for how you want it to turn out but not by much.
After spicing and herbing the fish insert the cooked asparagus into the cavity of the fish. Brush on melted butter that has been melted with garlic, either minced or granulated. (If granulated use a bit more than minced. Freshly mince if possible.) Wrap the salmon in aluminum foil to hold the asparagus in. (You can use some other method if wish if you want the salmon skin crisper.) I had to use foil as I cooked it in an electric skillet at the con and wrapping it in foil seemed best. It would cook faster covered. I wrap the salmon in aluminum foil when I cook on a grill too.
Alternatively you can steam the salmon with the asparagus outside the fish but it won't pick up the flavor of the fish. Cooking time depends on what heat you're cooking it at, and how hot your appliance cooks. Cooking at a lower heat will cook the asparagus more. Try 350-450 degrees (F). Check the salmon if need be to see if it is fully cooked and tender. How done it smells will give you some indication of when to check.
When you are ready to serve, have some additional melted butter with garlic for those that want to dip the asparagus and/or the salmon for extra buttery flavor. (Note: not recommended for those on low cholesterol diets.) I'm sorry for not giving exact measures and times. I never learned to cook that way. I learned to do it by eye and feel.
AS A GENERAL RULE, I use more spices and herbs than are commonly found in recipes in cookbooks. I've found that if you use more spices and herbs you get more flavorful food WITHOUT overpowering the flavor of the food. Yes, it is possible to overdo it! Learn by trial and error.