Spread out bread pieces on a rimmed baking sheet and let sit at room temperature until dried out, 1–2 days.
Place bread in a large resealable plastic bag and seal. Place in another bag, seal, and wrap in a kitchen towel. Using a rolling pin or meat pounder, smash to fine irregular crumbs.
Heat oil and butter in a medium skillet over medium until butter begins to foam. Add rosemary, thyme, and garlic; cook, tossing, until fragrant, about 1 minute. Add 1 cup breadcrumbs and cook, stirring occasionally, until golden brown, about 4 minutes. Transfer to paper towels and let drain. Remove herbs and garlic; season with salt. Let cool.
Breadcrumbs can be dried out and crushed 1 month ahead. Freeze airtight.
