For the garlic croutons: Preheat the oven to 375°F. In a large bowl, add the bread pieces and garlic and drizzle with enough olive oil to lightly coat the bread (about a couple tablespoons or so). Toss gently to combine and season with salt.
Place the croutons on a large rimmed baking sheet and bake for 20 minutes. Stir and continue to bake until the croutons are golden brown and crunchy, checking and stirring every 5 minutes.
For the meatballs: Preheat the oven to 450°F. Drizzle the olive oil into an aluminum foil-lined 9×13-inch baking dish and use your hand to evenly coat the entire surface. Set aside.
Combine the ground turkey, croutons, cranberries, eggs, bread crumbs, sage, salt, and cinnamon in a large mixing bowl and mix by hand until thoroughly incorporated.
Roll the mixture into round, golf ball-size meatballs (about 1 ½ inches), making sure to pack the meat firmly. Place the balls in the prepared baking dish, being careful to line them up snugly and in even rows vertically and horizontally to form a grid. The meatballs should be touching one another.
Roast for 20 minutes, or until the meatballs are firm and cooked through. A meat thermometer inserted into the center of a meatball should read 165°F.
Allow the meatballs to cool for 5 minutes in the baking dish before serving.