Place cubed focaccia in a large mixing bowl and drizzle with olive oil, dried herbs, and salt. Toss and stir to combine until all the pieces are somewhat evenly coated.
Drizzle the oil over the bread cubes instead of pouring it in all at once. It's almost impossible to get a truly even coating of oil, but you don't want the oil to all to absorb into one or two focaccia cubes. If you want to be particular about it, drizzle half the olive oil in and give the cubes a toss, then drizzle in the rest. You can always sprinkle in more herbs if you think they're needed.
Arrange the cubed focaccia in an even layer on a shallow, rimmed sheet pan. Give them a bit of breathing room so they're not all pressed up against each other.
Place the sheet pan in the oven and leave the croutons to dry. Stir or shake the pan every 15-20 minutes to help them dry evenly. This can take anywhere from 45 minutes to 2 hours, depending on the moisture content of the bread, how dry it was to start with, and how much oil it absorbed.
To test a crouton for doneness, remove one from the pan and let it cool slightly. Bite into it. When the center is completely dry and crispy, your focaccia croutons are done. If you're not sure if the croutons are done, give them another 10-15 minutes in the oven. The temperature is so low, they're not at risk of burning. You could even turn the oven off and leave the croutons in there as it cools for a long, slow drying period.
Let croutons cool completely before storing. To keep homemade croutons crispy, it is super important that they are completely dry inside. Store in an airtight container with a paper towel to absorb excess moisture for 2-3 weeks.
RECIPE NOTES
A bite-sized cube of soft focaccia bread is very different from a bite-sized cube of hard bread. When you cut the focaccia into cubes, cut them slightly smaller than you think they should be!
If using other seasonings, aim for about 2-2½ teaspoons total. Different seasoning blends have different intensities; adjust accordingly!
Dried rosemary leaves are big and pointy. Pinch and crush them between your fingers as you add them just to break them up a little.
Once your focaccia croutons are fully dried out and cooled, store them in an airtight container; you may find tucking a paper towel inside helps absorb any excess moisture. Focaccia croutons can be stored in an airtight container at room temperature for two to three weeks. You can also refrigerate them or freeze them for up to 3 months. If your croutons seem to be a bit soggy after a day or two in storage, they probably weren't fully dry when you took them out of the oven or the container wasn't fully sealed. Crisp them back up in the oven at 350°F for a few minutes before using them.
Other Focaccia Crouton Seasonings: Dried thyme, garlic powder, crushed red pepper flakes, parmesan cheese, nutritional yeast, seasoned salt, tomato powder. You can also use blends like garam masala, Old Bay, or Everything Bagel Seasoning (pulse it in a spice grinder to turn it into a powder first). If using a store-bought seasoning blend, check the ingredients; if salt is high up on the list, you may not want to add more salt to your croutons.