Add the avocado, yogurt, mayo, garlic, arugula, herbs, chives, lemon juice, salt and pepper to a food processor and blend until combined. You want the mixture to be creamy and spreadable!: The moment you blend these ingredients you will notice a fresh, herbaceous aroma rising, with the garlic giving an immediate savory edge. The texture should be silky and spreadable, with tiny flecks of green visible from the parsley and basil . This velvety consistency helps the dressing cling evenly to the tuna so each bite is balanced; if it is too thick add a teaspoon of water or lemon juice to loosen it slightly. A common mistake is overblending which can warm the mixture and mute the fresh herb flavors, so pulse just until combined and scrape down the bowl to ensure even texture.
To make the melts, add the tuna to a large bowl. Add the baby arugula. Add about 1/4 cup green goddess dressing to start and toss the mixture together until everything is combined.: As you flake the tuna into the bowl you will see moist, distinct pieces that will soak up the dressing without becoming mushy. Folding in the fish gently preserves its flaky texture, which contrasts nicely with the creamy dressing. Drain any excess liquid from the canned tuna beforehand to prevent watery filling. Avoid vigorous stirring that can turn the fish into a paste, because that will change the intended mouthfeel of the tartines.
Spread a spoonful of the green goddess dressing on each piece of toast. Top with the tuna mixture. Top each with about 1/2 ounce of havarti cheese. Heat the broiler in the oven to high and place the tuna toasts under for just 1 to 2 minutes, until the cheese is melty. Serve immediately with potato chips or whatever side you'd like!: Tossing in the extra cup of fresh arugula adds bright peppery notes and a lightly crisp texture. The greens should remain slightly lively and not wilted, offering a fresh contrast to the creamy components. Mix just enough to distribute the leaves evenly; overmixing will bruise the arugula and make the filling soggy. A typical error is adding unwashed greens that bring extra moisture and grit, so make sure they are well dried.
Add about 1/4 cup green goddess dressing to start and toss the mixture together until everything is combined: Starting with about 1/4 cup of the dressing lets you control the coating so the mixture is moist but not drenched. You should see the dressing lightly gloss the tuna and leaves, with small pockets of creaminess rather than a pool of sauce. Add more dressing incrementally if needed, tasting as you go to balance salt and lemon. One mistake is adding the full amount at once which can overwhelm the texture, so work in stages for the best result.
Spread a spoonful of the green goddess dressing on each piece of toast: When you spread the dressing onto the toasted multigrain bread you will feel an instant contrast between the crisp toast and the creamy layer. This base layer protects the toast from becoming soggy while infusing each bite with herb flavor. Use a moderate spoonful and spread evenly to the edges for consistent flavor. Rushing this step and piling on too much filling can collapse the toast and make it difficult to eat neatly.
Top with the tuna mixture: Pile the dressed tuna onto each toast so that every bite includes both fish and greens. The filling should sit slightly proud of the toast rather than overflowing, which helps it heat evenly under the broiler. Press gently to ensure contact with the toast, which helps melding during the brief broil. A common pitfall is overstuffing the toast, which can cause the tartine to slide apart when you pick it up.
Top each with about 1/2 ounce of havarti cheese: Sprinkle grated havarti cheese evenly over the top so it melts into a glossy blanket. The cheese adds a creamy, slightly tangy counterpoint and helps bind the topping. Grating the cheese fresh ensures it melts uniformly; pre-sliced or thick chunks may not melt as quickly. Avoid using too much cheese which can overpower the herb flavors; a light even layer is best.
Heat the broiler in the oven to high and place the tuna toasts under for just 1 to 2 minutes, until the cheese is melty: When broiling you will hear a faint sizzling as the havarti cheese softens and begins to brown at the edges. Keep the oven door slightly ajar if your broiler allows, and watch closely because the window is short. The ideal visual cue is bubbling cheese with tiny golden spots, which signals readiness. Leaving them under the broiler too long will scorch the cheese and dry the tuna , so never step away during this step.
Serve immediately with potato chips or whatever side you'd like: Serve the tartines right away so the contrast between warm melted cheese and cool herbaceous dressing remains pronounced. The crunchy toast will still have a pleasant crispness and the filling will be warm without being hot, which preserves the fresh herb character. If you wait too long the toast may soften and the greens can wilt, so plan to plate as soon as the cheese melts.