Remove vegetables from freezer and thaw, preferably overnight. Otherwise, place vegetables in a colander and rinse with cold water. Place colander over a bowl to drain thoroughly. Once drained, spread vegetables on paper towels.
in a large bowl, whisk together eggs and milk until well combined. Season with salt and pepper. Add bread, vegetable mix, baby spinach, sliced garlic, cheese (reserve a handful to sprinkle over top) and mix until well combined. Transfer to a buttered 9-by-13 inch baking dish, cover with plastic, and refrigerate overnight.
When ready to bake, heat oven to 325°F. Remove plastic and bake, uncovered, about 35 minutes or until tester inserted in center comes out mostly clean. Sprinkle grated cheese on top, turn on broiler to high, and broil on top rack, checking after 2 minutes, until cheese is melted and top is golden brown and bubbling. Remove from oven and let sit at least 10 minutes before serving.
You can use leftover or stale bread that has been left out overnight to dry. Sliced sandwich bread will also work, but the best bread is one that has a chewy bite or large air holes and a thicker crust. If you choose to use fresh bread, cut it into 1 inch pieces and toast in a low oven (275°F) until fairly dry.