Go into edit mode, select everything and hit F to fill the circle
Hit E to extrude and give your disk the right thickness
Switch to Face mode by hitting 3 on your keyboard (not numpad)
Select both of the large circular faces top and bottom
Hit Ctrl+B to Bevel, move your mouse to the right. Hit C on your keyboard to Clamp the bevel and stop it overshooting, scroll up on your mousewheel to increase the resolution of the bevel. Click when happy and then select All with A and remove any doubles with Alt+M and By Distance.
Tab into Object mode, right click on object and Shade Smooth
Another way is using the Spin tool.
Start with a circle the same size as you want for the semicircle curve around the edge.
Make sure it has an even number of vertices with one vert at the top and one at the bottom and enough points to make the edge rounded enough.
Delete the verts on one side, but not the top and bottom points, leaving the semi circle for your outer edge. Move this over by the radius of your final shape. Then use the Spin tool to spin it around its origin by 360 degrees with as many points as you want for a nice circle.