In a small bowl, whisk the vinegar and mustard. Season with salt and pepper. Gradually whisk in the oil.: The moment you start whisking you should smell the sharp, tangy lift from the balsamic vinegar meeting the pungent notes of the whole grain mustard . As you whisk, the mixture will turn slightly thicker and glossy, which is the visual cue that the two are marrying. Work quickly and steadily so you emulsify before adding the oil, and keep an eye on texture, because if you overwhisk the mustard seeds can break down too much. A common mistake here is adding oil too fast, which prevents a smooth emulsion; to avoid that, pour the vegetable or canola oil in a thin, steady stream while whisking.
Place the lettuce and spinach in a medium salad bowl and toss together. Core the pears and apples and cut them into 1/2-inch dice. Place in a separate bowl and toss with the lemon juice. Add the pears and apples to the salad bowl along with the grapes, Gruyere and dressing; toss.: Right after the dressing starts to form, adding a pinch of kosher salt and a few grinds of freshly ground black pepper enhances aromatic complexity. You should be able to detect a rounded saltiness that brightens the vinegar and tames the mustard's sharp edge. Taste and adjust gently, because salad greens will absorb seasoning, and oversalting at this stage is easy to do. If you go heavy on salt, balance it with a touch more balsamic vinegar or lemon juice.
Sprinkle the toasted pecans on top, and serve.: As you stream in the vegetable or canola oil , the dressing should thicken and become silky, coating the back of a spoon. When the dressing looks cohesive and slightly viscous, you know it is well emulsified. Listen for the slight glug of oil hitting the bowl, and watch the dressing pick up sheen. Adding the oil too quickly will separate the emulsion, producing an oily slick and a cloudy dressing. If separation occurs, start with a fresh small bowl and whisk in the separated mixture slowly into a new mustard base.
Place the lettuce and spinach in a medium salad bowl and toss together: When you combine the chopped Boston lettuce and chopped spinach leaves , you will see contrasting shapes and colors; the light green of the lettuce and the deeper green of the spinach create visual appeal. Toss gently so leaves do not bruise and they get light air pockets for the dressing to cling to. If you rush and overwork the leaves they can become limp, so use a light folding motion. A common error is dressing too early; dress only after the fruit is prepared so the greens stay crisp.
Core the pears and apples and cut them into 1/2 inch dice: After coring, watch for the fruit's aroma as you slice, a fragrant sweetness that signals ripeness for the Anjou or Comice pears . Cutting into uniform 1/2 inch cubes ensures even mouthfuls and consistent texture. If you cut pieces too large they may overwhelm the greens, and if too small they can become mushy. Keep a sharp knife to make clean cuts, since dull blades bruise the fruit and accelerate browning.
Place in a separate bowl and toss with the lemon juice: As soon as the fruit is diced, tossing it with the freshly squeezed lemon juice brightens the flavor and prevents discoloration. You will notice the citrus aroma lift the pears and apples, and the juice forms a thin, protective sheen on their surface. Work quickly so the fruit is coated before oxidation sets in. One mistake is letting the fruit sit too long after cutting; always toss right away to keep the color and texture pristine.
Add the pears and apples to the salad bowl along with the grapes, Gruyere and dressing; toss: When you combine the fruit with the greens and Gruyere cheese , toss gently so each element is evenly distributed and coated with dressing. You should hear a light shuffle as the grapes and cubes move through the leaves, and visually the bowl should show specks of cheese and halved seedless grapes . Tossing too vigorously can bruise the fruit and crush the grapes, releasing too much juice; use a folding motion to maintain integrity.
Sprinkle the toasted pecans on top, and serve: Finally, scatter the toasted pecan halves over the dressed salad so they sit on top and retain crunch. The warm, toasty aroma of the nuts will be noticeable if you toasted them recently. Serve immediately so the pecans stay crisp and the greens do not wilt under the dressing. A common pitfall is adding the nuts too early, which makes them soggy; always add them at the last moment.