Step 1Whisk together the whole-grain and Dijon mustards, the chopped thyme, the water and 1/4 teaspoon pepper in a large bowl. Whisk in the oil. Set aside half the mixture for grilling. Place the chicken in the bowl and toss to coat. Cover and let sit at room temperature 30 minutes (or refrigerate 2 hours or overnight).
Step 2Heat the grill. Season the chicken with salt and pepper. Lay the chicken breasts on the grill, skin side down, and cook, covered, until well seared, about 8 minutes. Turn, brush with the reserved marinade and grill until crisp and cooked through, 10 more minutes. To serve, garnish with the almonds and thyme sprigs and season with salt.
Roasting: Heat the oven to 500 degrees. Lay the chicken in a roasting pan, skin side down. Roast, turning once and basting with the marinade, until the chicken is crisp and cooked through, about 25 minutes.