Spread the bread cubes in a single layer on a parchment-lined sheet pan. Let them air-dry at room temperature for up to 2 days, tossing occasionally, or dry them in a 350°F oven for about 15 minutes, tossing once halfway. Toss so all sides dry evenly. The cubes should be crisp and dry but still pale.

Pulse the dried bread in a food processor or blender until you have coarse crumbs of mixed sizes, from pea-size and smaller. Air-dried bread can also be crumbled by hand. You need enough to yield 1½ cups of crumbs.

If the crumbs are not fully dry at this point, spread them on the sheet pan and toast briefly in the oven. Cool completely before proceeding.

In a large skillet over medium heat, melt the butter in the olive oil until foamy. Add the crumbs, onion powder, garlic powder, and salt all at once and stir until every crumb is coated. Reduce the heat to medium-low.

Stir constantly until the crumbs are an even golden brown and smell toasty, about 3 to 5 minutes. The smallest crumbs brown first, so pull the pan when most are golden.

Immediately pour the crumbs onto a parchment-lined sheet pan and let them cool completely. They will crisp as they cool.

Store in an airtight container at room temperature for up to 1 week.