It really is disappointing to discover the damage.

Since leaf miners overwinter as pupae in the soil and in plant tissue, I would avoid replanting in that area or wherever you had garlic planted previously. Think about 3 year bed rotations.  The adults are going to be emerging this month or into May to lay eggs, so a new bed  planted with onions (after mid-May)  should be covered immediately.  This goes the same for Fall--plant garlic/leeks in a new area and cover with insect proof mesh as the second generation of allium leaf miners is flying and laying eggs (Sept-Oct).  Definitely dispose of the damaged plant residue and anything that was below ground rotting in the soil--may have allium miners in it and in the soil.

Penn State recommends cover crops from the brassica family  being grown in the fall planting bed prior to putting in your alliums due to the sulfur from those plants transferring into alliums which apparently repels the allium leaf miners.  Also recommend avoiding high Nitrogen fertilizers which attract the pest.

You will also note our UMD page gives options for chemical controls if it comes to that.

https://extension.umd.edu/resource/allium-onion-leafminer/