Preheat your oven to 350ºF. Heat the olive oil in a deep sauté pan over medium heat and sauté the onion until it starts to soften, about 5 minutes.
Add in the frozen green beans and 1/2 teaspoon of salt and stir well. Cover the pan and let the vegetables cook until heated through, about 8 to 10 minutes. The beans won't become any more tender in the oven, so make sure they are as soft as you want them to be before moving on.
While the vegetables are cooking, combine the drained beans, 1/2 cup of water, garlic, 1 teaspoon of salt, rosemary, a couple grinds of fresh black pepper, and the lemon juice into a blender. Blend until smooth. Add 1/4 cup more water for a thinner sauce. (If using chickpeas, you might even need more water, but add it only 1/4 cup at a time.)
To prepare the topping, use a food processor or blender to pulse the walnuts and salt together, just until coarsely ground. (It's doesn't need to be super-fine, since you want some crunch!)
When the green beans are tender, pour the sauce over the top and stir well. Transfer the mixture to an oven-safe casserole dish, if you need to, or you can use the same pan if it's oven-safe. Adjust any seasoning to taste, then sprinkle the crushed walnut mixture over the top. Bake the casserole at 350ºF for 30 minutes, until the vegetables are piping hot and the topping is lightly golden.
Allow the casserole to cool for 10 to 15 minutes, then serve warm. Leftovers can be stored in an airtight container in the fridge for up to 4 days.
Nutrition information is for 1 of 6 servings. This information is automatically calculated, and is just an estimate, not a guarantee.I've tested this recipe with both Great Northern Beans and chickpeas, so it works with both! I find that you need a little more liquid for blending when you use the chickpeas, so feel free to add more, as needed until the sauce is smooth and easy to pour.You can use balsamic vinegar instead of lemon juice in this recipe. It's delicious either way!Make Ahead Tips: You can assemble the cooked green beans and sauce ahead of time, and store them covered in the fridge for up to 3 days in advance. When you're ready to reheat it, add the walnut topping and bake at 350ºF until heated through, about 35 to 40 minutes.Warning: Do not place a cold glass dish in a hot oven, or you may risk shattering the glass. Instead, transfer the make-ahead casserole to a glass or ceramic dish that has been at room temperature right before baking, or use a metal pan.