I have a similar body type. 5’10” and range from 140 to 148lbs. I have always been lean and found it hard to gain weight. The only thing that worked in the past was consistent weight training 4days a week and consistently eating more meals and calories than usual. About a year ago I was having GI issues and after some tests I was diagnosed with Celiac disease. I think this along with increased training load from cycling dropped my weight down to 140lbs at times.
Now it is the off season and the weather is bad so my training has moved in doors. I am doing a small amount of weight lifting and now doing shorter more focused workouts on TR starting with Sweet Spot Base. I decided to gain the weight back that I lost because I want to grow my leg strength and I know I can and will lose weight during the spring. I think my legs are more of a limit than my cardio fitness on longer rides so the idea is to change that. I do well on climbs but get hurt on the flats so I’m targeting my weakness in that way.
My method is a combination of the training mentioned above and the application of what I have learned about which foods work for my body. I start with a huge breakfast meal that most people would consider for lunch or dinner. This is usually a full plate with rice and meat or eggs and sometimes with a gluten free waffle, pancake, or cereal. Then I add a smoothie with blueberries, raspberries, avocado, blackberry, banana, spinach, chia seeds, flax meal, and honey for extra calories. I pack a slightly smaller lunch to work consisting of leftovers that my wife cooked and some fruit and water. If I am doing a workout that day, I try to have a second smaller lunch around 3pm. Dinner is usually my smallest meal and I try not to eat within two hours of bed time because it disturbs sleep. I also drink recovery shakes right after rides and pack a meal to a ride that is more than 30 minutes from home. So far I’ve gained about 4lbs in 4 weeks and I feel strong. I plan to cut back if I get to 150 lbs as that would be my heaviest since I did a lot of weight training.
If you want to gain muscle mass, look at what power lifters and body builders do. They lift two or three times a day and eat all day… They are basically force feeding all day long at times. If you can apply this concept with healthy food and quality workouts, you will get strong. It does take time though. You can probably gain less than one lb of muscle mass per week. So at least 8 weeks to gain 6 lbs of muscle if you are meticulous.