First, you're tasting a warm sample. It's not properly carbonated and chilled yet, so the perception of sweetness is much higher. RDWHAHB may apply.
Did you take a gravity reading (FG) after fermentation was done?
What are your OG and FG? What yeast did you use?
What style is it?
Where did you source the recipe? Maltiness, sweetness, dryness, bitterness, hoppiness, and alcohol content are main factors that need to be in some sort of balance to have a good beer in hand.
Most recipes are designed with those parameters in mind. You reduce one of those drastically in such a recipe, the beer becomes unbalanced, which is probably what happened here.
Now you can have extreme beers, like 200 IBU, or an FG of 1.030, or 30% alcohol, or using a pound of dry hops in a 5 gallon batch, etc. Yet those main factors (and other ones) all weigh in to create some form of balance that makes it good (although not necessarily to everyone's taste).
Hops do 2 main things for taste perception:
- Bitterness
- Flavor and aroma
As you already know, bitterness is created by boiling hops for some time. The stronger the AA% in the hops, the more you use, and longer you boil, the higher the bitterness. The longer you boil, less the less aroma and flavor are left. To create flavor and aroma, those hops are usually not boiled at all (steep/hopstand/whirlpool and dry hopping) or not for very long (late boil and flameout additions).
Here are 3 possible routes:
1. Null option, do nothing. Just enjoy the beer for what it is. Chalk it up to experience.
2. If you just need more hop flavor and aroma, a dry hop will fix that, may even give you a little bitterness.
Think of a more strongly hop flavored, but not very bitter ESB, Pale Ale, British IPA, etc. Or one of those modern day NEIPAs.
3. If it really lacks bitterness, you could indeed raise it a little by making a strong bitter potion and add it to your beer.
But there are some pitfalls and limitations.
First, I would not boil any of the beer.
Second, if you want to add 20 IBU to your 2 gallons (8 quarts) of beer, you can boil, for example, an additional 1 quart of water (wort)* with some high AA% hops for an hour to create wort with 180 IBU:
180 IBU / 9 qts = 20 IBU
or more realistically, since 180 IBU is near utopia, boil 2 quarts to 100 IBU:
200 IBU / 10 qts = 20 IBU
Please mind, the more potion you add the more you'll dilute your original beer.
* You really should boil hops in wort not water. You could add just enough malt extract (or sugar) to those 2 quarts so your 2.5 gallons of beer will carbonate from them, just right. Add that potion right before bottling, don't add any (extra) priming sugar.
During the boil (a good simmer is enough, it doesn't need to be a rolling boil) some water will evaporate, effectively concentrating your wort. You don't want that (it reduces hop utilization). So keep topping up every 5-10 minutes with some boiling water (keep a pot simmering on low on the side) to keep it at 2 quarts for that hour.
Let us know which path you choose...
