Easy.

1.) Use the crossbow as a threat to coerce others into growing spinach for you, using your initial stock as a starting point.

2.) Once their first spinach crops mature, collect a tax from each farmer at crossbowpoint to buy yourself a gun and ensure their future compliance. Sell the remainder of this crop to fund the purchase of weaponry for each of them.

3.) Have each farmer coerce 2 more farmers and so on down the line. Iterate until your spinach empire is self sustaining and producing $1Bn in revenue. Proper scaling and business methods are left as an exercise to the reader.

4.) Hire Elon Musk and the entire departments of physics at MIT, Caltech and Stanford for your SpinachX team and put them to work designing a time machine.

5.) Go back in time and provide yourself spinach and a crossbow. Trust me, you'll thank yourself later.