The best way to machine an open 2D pocket in Fusion is to use the 2D Adaptive Roughing strategy.
To machine an open 2D pocket using the 2D Adaptive Roughing strategy, follow these steps:
On the Manufacture workspace toolbar, click Milling > 2D > 2D Adaptive Roughing
.
The 2D Adaptive Roughing dialog opens.
On the Geometry tab, click Select next to Pocket Selections.
In the Pocket Selections table, open the drop-down list in the row and choose Pockets, then select the open pocket on the canvas.

Selecting the open pocket
Adjust the linking heights so that they match the geometry.
Ensure the top of the stock is above the cutting depth.
Click OK to generate the toolpath.
The toolpath should now look similar to the image below:

2D adaptive toolpath for an open pocket