Step 1

In a medium (4- to 6-quart) pot, combine the potatoes, cauliflower and garlic, and add enough cold water to cover by about 1 inch. Bring to a boil over high heat, then reduce the heat to medium-low and simmer until the vegetables are very tender, 12 to 15 minutes. Drain, then transfer the vegetables to a large bowl. Add the broth, 3 tablespoons of the oil and 3/4 teaspoon of the salt, then use a potato masher to mash to your desired consistency.

Step 2

In a medium skillet -- about 10 inches -- over medium heat, heat 2 tablespoons of oil until shimmering. Add the onion and cook, stirring, until slightly softened, about 2 minutes. Add the spinach, 1/8 teaspoon of salt and the pepper, and cook, stirring, until the spinach has warmed through and any remaining liquid has evaporated, 1 to 2 minutes.

Step 3

Position a rack in the upper third of the oven and preheat to 350 degrees.

Step 4

Brush a 6-cup casserole dish with olive oil. Layer half of the mashed potato-cauliflower mixture on the bottom. Then add the spinach mixture on top, spreading it out as evenly as possible. Top with the remaining potato-cauliflower mash.

Step 5

In a small bowl, toss together the matzoh meal or panko with the remaining 1 tablespoon of oil and 1/8 teaspoon salt. Sprinkle over the top of the casserole and bake for about 1 hour, or until the top is lightly browned and the casserole is hot in the center. Serve hot or warm.