I usually start my workout week with a leg workout. For me, after a few days from the last week's workout, I would have enough energy to maximize my leg workout. Leg workout is important because it is a huge muscle group and plays a big role with production of testosterone. Testosterone is a hormone that helps build muscles. In other words, I find that working legs in the first day of the week is my strategy to build muscles.
Typically, as someone who is a hard gainer, I only spend 40 mins in the gym. When I was a beginner, I thought that staying in the gym for 2 hours will help me build muscles faster; however, this is not the case for someone who is underweight. A smart technique is to only to workout enough to stimulate muscle growth. So far, here is my staple workout for my leg day:
Barbell Back Squats 6 sets of 6's
Bulgarian Split Squats 4 sets of 8's
Goblet Squats 4 sets of 8's
(I may also throw in a core workout.)
I also incorporate lots of leg workout throughout the week. Usually I start my workout session with some leg training to warm myself up.
For most people starting out in the gym, it is famous to focus training the biceps or chest. But if you are looking to save plenty of time, start investing in learning how to do those squats. Train those legs and you will realize you do not even need performance enhancing drugs (i.e. steroids). For my personal experience, I usually feel good after a good leg day workout.
Don't skip the leg day or you'll look like martini glass (wide upper body but skinny legs).