Make the base. Line a regular loaf pan with plastic wrap. Set aside.
Combine the chia, almonds, cashews, and salt in the bowl of a food processor and process until the mixture looks like a fairly fine meal (no pieces of almond should be visible). In a small bowl, mix together half of the lemon zest, half of the lemon juice (about 2 tablespoons or 30 ml), the agave nectar, stevia, and vanilla until everything is well combined. Pour the lemon-juice mixture over the dry ingredients in the processor and blend until it comes together in what looks like a sticky dough (if it's too dry, add up to 1 tablespoon or 15 ml water). Sprinkle with the poppy seeds and pulse just until they are incorporated. Turn the base into the prepared loaf pan and, using wet hands or a silicone spatula, press it down firmly and evenly. Place the pan in the fridge while you prepare the frosting.
Make the frosting. In the bowl of a food processor or using a hand blender, blend the chia, coconut oil, nut butter, reserved lemon zest, reserved lemon juice (you should have about 2 tablespoons or 30 ml), and stevia until perfectly smooth. The mixture may liquefy as the coconut oil melts; this is fine.
Frost and unmold. Pour the frosting over the base in the pan and swirl the top. If desired, sprinkle with additional lemon zest and poppy seeds to garnish (see photo, page 208). Refrigerate until firm, at least 2 hours. (Tip: If the frosting is too liquid to hold a shape when you first pour it over the base, refrigerate about 15 minutes until it firms up a bit, and then add any swirls that you like.) Once the top is firm, fold the plastic wrap over it to cover.
To unmold, peel back the plastic on top and invert onto a cutting board; turn right-side-up and cut into 12 or more pieces (they should be relatively small). Serve immediately; store leftovers, covered, in the refrigerator, for up to 5 days. May be frozen. Defrost in refrigerator. Makes about 1 dozen.