You want to build resp. increase muscle mass and build up muscle, then an optimal diet is absolutely necessary next to your workout.
The diet must ensure that the muscles get the material they need to grow.
For muscle building as well as for intensive training there must be provided sufficient building materials (nutrients) and energy. Therefore, the diet should be based on three main meals and every two to three hours there must be small meals inbetween. The daily energy intake should be increased to at least 500 calories more (calorie surplus).
In order to develop enough strength in training and to set the necessary growth stimulus, energy-rich carbohydrates (i.e. potatoes, pasta, rice, whole grain bread, etc.) are important. Muscle growth needs protein. High quality protein is found in meat, fish, eggs, legumes and dairy products.
To increase the biological value for the body, the combination of plant and animal protein (such as potatoes and eggs) is useful.
For vegan protein supply, rice protein is very well suited.