Adjust the oven rack to the middle position and preheat the oven to 425°F (220°C).

Grease a 9x13-inch (23x33 cm) baking dish with a small amount of coconut oil.

In a mixing bowl, combine the 15 oz (425 g) of tuna, 0.7 tbsp (10 ml) olive oil, 0.7 tbsp (10 ml) lemon juice, approx. 0.1 tbsp sea salt, and 0.1 tbsp pepper. Flake the tuna into coarse pieces with a fork and let it sit for 10 minutes.

Julienne the zucchini to create “noodles. ” Place the 12 oz (340 g) of noodles on a lightly greased baking sheet.

Sprinkle lightly with sea salt and pepper.

Place in the oven and bake for 5 to 10 minutes, or until the noodles release much of their moisture but remain firm (not burned).

Remove and set aside. (Note: If using prepared spaghetti squash instead of zucchini, skip this oven step).

Over medium-high heat, heat 0.35 tbsp (approx. 5 g) of coconut oil in a nonstick skillet until shimmering.

Add the sliced mushrooms and chopped onion.

Cook until the liquid evaporates and the mushrooms are browned, typically 5 to 7 minutes.

Whisk in the 24 fl oz (710 ml) of broth and 12 fl oz (355 ml) of coconut milk, scraping up any browned bits from the pan, and bring to a simmer.

In a small bowl, combine the 4 fl oz (120 ml) of water and 2 tbsp (16 g) of arrowroot or tapioca flour.

Whisk this mixture into the pot.

Continue to simmer, whisking constantly, until the sauce is slightly thickened, about 3 minutes.

Remove the skillet from the heat.

Whisk in the 7 oz (200 g) of cheese, a handful at a time, until fully melted.

Stir in the cooked zucchini noodles (or spaghetti squash), the seasoned tuna mixture, the drained spinach, 0.2 tbsp sea salt, and 0.2 tbsp pepper.

Transfer the mixture to the prepared baking dish.

In a small bowl, toss the 3 oz (85 g) of toasted breadcrumbs with the remaining 0.35 tbsp (5 g) of melted coconut oil.

Season with salt and pepper to taste.

Sprinkle the breadcrumb mixture over the top of the tuna mixture.

Bake until the sauce is bubbling around the edges, approximately 12 to 14 minutes. Allow the casserole to cool slightly before serving.

Don't Skip the Pre-Bake: The secret to avoiding a watery casserole is handling the zucchini correctly. Zucchini is mostly water! That pre-baking step is crucial to draw out moisture before it hits the sauce. If they still look "weepy" after coming out of the oven, pat them gently with a paper towel before adding them to the skillet.

The "Slurry" Secret: When adding the tapioca or arrowroot flour, always mix it with the cold water first to create a smooth liquid (a slurry) before dumping it into the hot pan. If you toss dry powder directly into the hot broth, it will clump up instantly and be impossible to smooth out.

Drain the Tuna Aggressively: "Drained thoroughly" means drained thoroughly. Press the lid of the can down into the sink to squeeze out every last drop of water. Any extra liquid from the tuna will dilute your creamy coconut sauce.

Cheese Melting Magic: Dairy-free cheeses (like the Coconut Milk Cheddar used here) melt differently than traditional dairy. They often take a little longer or need lower heat to avoid becoming oily.

Stir it in gradually off the heat, just as the recipe states, for the smoothest texture.

Storage Savvy: Because arrowroot and tapioca are starches that can change texture when reheated, this dish is best eaten fresh. However, if you do meal prep, store the sauce/tuna mixture separately from the zucchini noodles and combine them just before baking to keep the texture fresh.