for instance
5 - 7 eggs and bacon for your sandwich (you can toast them if u like) and ditch the milo

the additional eggs are just recommendation, of course the other meals gotta reflect the necessary changes as well-  more protein more good fats

U are really underweight, it's okay to push the protein consumption to 1.5g-2g per lb of bodyweight and let the others (carbs and fats) take care of itself after establishing your baseline calories needs

some ways to establish (please note this may not be exactly accurate but it presents a good start to work with)

the quick method

Use this formula for a personalize ballpark estimate. use the lower number for lightly active, the middle number for moderately active and well higher for very active

Muscle Gain
16-18 calories per lb. of BW

Maintenance
14-16 calories per lb. of BW

Fat loss
12-14 calories per lb. of BW

or

The harris benedict formula

or

The Kathe-McCardle Formula

U gotta do some work and google the last two if u are interested. Ultimately just pick one and stick with it, if u r not seeing weight gain +250 calories every week from the baseline.

and I don't really understand the whole idea of Mass Gainer ??

Say ON serious MASS

12lbs Chocolate
Serving Size: 2 Heaping Scoops (334g)
Servings Per Container: 16

for RM 230 for 16 servings ?!

Each serving yield a paltry 50g of protein, 252g of carbs in the form maltodextrin, 4.5g fat.

Any generic whey protein brand for 10lbs cost about 270 - 330 =approx 160 servings of 22 - 24 g of protein. M i missing the math here ?!!!

U can add your carbs or follow the (1,111 calorie shake) i posted or if u want more ideas for home made mass gainer , let me know and i will post more.

I hope this clears out most questions for gaining weight