Preheat your oven to 400°F / 200°C and line a baking sheet with parchment. Place the squash, cut side down, on the baking sheet. Bake for 40-50 minutes, or until the squash is completely tender and a knife can easily pierce through the skin. Remove the squash from the oven and allow it to cool until it's cool enough to handle. Slice the squash into 1/2-inch / 1.3cm thick slices.
While the squash roasts, bring a few inches of water to boil in a medium pot, then fit the pot with a steamer attachment. Steam the kale for 4-5 minutes, or until tender but still bright green. Set the kale aside.
In a small bowl, whisk together the Dijon mustard and agave or maple syrup.
When the squash is cooked and sliced, scoop the avocado flesh out of the skin and transfer it to a small bowl. Use a fork to mash it lightly. Add freshly squeezed lemon juice, salt, and pepper to your liking.
Toast your bread, if desired. To assemble sandwiches, spread 2 teaspoons of the agave mustard mixture on one slice of bread. Top with the other slice of bread with a quarter of the mashed avocado, using a fork to spread the avocado around. Top the avocado with a quarter each of the squash slices and the steamed kale. Close the sandwich with the slice of bread with mustard on it. Repeat this process for the remaining sandwiches.
Slice the sandwiches in half and enjoy.