Preheat the oven to 350° Fahrenheit. Choose a large casserole dish with a lid.
Bring water to boil in a small saucepan. Turn off heat. Add honey, stirring to dissolve. Add rosemary and cover to steep for two hours. Remove any twiggy rosemary branches. Stir in optional Dijon mustard, until fully incorporated.
Place chicken in large casserole dish. Spoon rosemary syrup over chicken. Drizzle olive oil over chicken. Season with salt and pepper. Cover and place in preheated oven for 1 hour, until juices run clear when poked deeply with a knife. Remove chicken and turn oven heat down to 325 degrees Fahrenheit.
Meanwhile, after first placing chicken in the oven, wash cauliflower and remove outer green leaves. Place whole cauliflower (uncut) into a steamer basket over simmering water. Allow cauliflower to simmer or steam until very fork-tender, about 40 minutes. Carefully transfer it to an oven-proof bowl to cool slightly.
Break cauliflower into small chunks with a spoon. Transfer ⅓ to ½ of the cauliflower to a blender. When the chicken is done cooking, use the gravy-like pan juices. Measure out about ⅓ to ½ the total pan juices, enough to puree the cauliflower to a creamy "baby food" consistency. Also add the ½ teaspoon sea salt to the blender. Keep the chicken dish uncovered* and place it back in the oven while you prepare the cauliflower. Puree cauliflower till smooth, adding more pan juices as needed.
Stir puree back into bowl of small cauliflower chunks. Place this bowl into the warm 325° oven. Allow both chicken and cauliflower to get toasty hot and a wink toasty, 10 minutes more in the oven. Serve with extra pan juices spooned over the top of both.
*For GAPS Introduction Diet, keep the chicken covered while it heats.