Preheat the oven to 400 °F and grease a 9x13 baking dish.
Chop all ingredients. Slice the focaccia bread into 1-inch cubes.
Blend the flaxseed with the water and allow to hydrate until it's a thicker gelatinous consistency.
Heat the olive oil in a large skillet over medium-low heat, and add the onion, fennel, and apple, with a big pinch of salt. Cook for about 3 to 4 minutes, stirring occasionally. Then, add the thyme, basil, and spinach leaves. Mix everything together and cook for one minute and a half more. Take off the stove and add the walnuts.
In a large bowl, mix together well the vegetables, focaccia cubes, and flaxseed. Season with salt and pepper to your taste. When everything is combined, transfer over the prepared baking dish.
Bake for 20-25 minutes, until the stuffing is golden brown and crisp on top. Finish the dish with some fresh parsley and enjoy.