There are two types of recipes you can use in your Workshop; Ship Upgrade Recipes and Fusion Recipes
In order to unlock Ship Upgrade Recipes, you must research them in the "Hull Upgrades" tab of the Ship Lab. Once they are researched they will show up on the Crafting Tab in your Workshop.
Fusion Recipes are a little trickier as they are not researched, and are obtained differently depending on the type or recipe.
-Core to Core recipes; allows you to fuse multiple lower tier cores in one higher tier core. These are unlocked by finding at least one of the higher tier cores you are trying to craft. Once you discover a Core to Core recipe it will stay on the Fusion Tab indefinitely.
-Pattern to Core recipes; allows you to take a higher tier ship pattern and break it down into a lower tier core. This will pop up on the Fusion tab in your Workshop once you find a Pattern that can be broken down into a core, but will disappear if you use all of that specific pattern up. Once you find more of that pattern the recipe will reappear on the Fusion Tab.