if you want 20lbs in a month of muscle sounds like you need steroids though I wouldn’t recommend that.

if you want to be healthy and put on as much weight as possible you need to increase your calories a lot. Remember you need to consume 3500 calories to put on 1lbs of fat. though if you want to gain muscle you have to lift correctly eat healthy foods and a lot of them. It’s hard to say without doing a complete analysis on your body and your nutrition and everything that goes into designing a custom program for amazing results like you seek. Here are some basic recommendations.

Eat 3500-5000 calories a day, build up to this each week increasing your calories though if your short on time go for it right away, though I warn you, you may be a bit full and feel sick.

For Breakfast eat:

5 Free range organic eggs (cooked in butter order from tropical traditions)

1 cup of steal cut oats

mix 1/2 cup of sunflower butter (avoid peanut butter)

for mid morning snack:

1 cup of brown rice

3 cups of a veggie of your choice

2 chicken breasts

next snack

Protein shake

2 tbs of olive oil

2 tbs of cod liver oil

1 big scoop of almond butter

add 1/4 cup of ground fax seeds

add 1/2 cup of coconut flower

1 cup of frozen fruit

lunch:

eat 8 oz steak

Large Salad

1 cup beans

1 cup of brown rice

snack

eat 5 free range organic eggs

2 cups black beans

dinner

8 oz peace of fish (cooked in butter)

1 cup of brown rice

3 cups of any veggies

2 tbs of olive oil

wake of in middle of the night and eat:

Protein shake (casinate (milk protein))

2 tbs of olive oil

2 tbs of cod liver oil

1 big scoop of almond butter

add 1/4 cup of ground fax seeds

add 1/2 cup of coconut flower

1 cup of frozen fruit

I don’t recommend trying to add 20lbs this fast though here is the answer to your question. It is very extreme and wouldn’t be recommended by myself though I wanted to give you an answer to your question so anyone who wants to add weight fast can do this plan though they do it at their own risk. Let me know how this turns out.

you can exchange protein sources such as streak for fish, chicken, turkey, ect. and vs versa.